Roaming is an important and often overlooked role on a team. It helps keep all of your lanes farming effectively and pretty much makes it so that you have 3 solo lanes (assuming you have a jungler). The role of a roamer is to keep all of your teammates alive and farming while obstructing the enemy farm or even killing them. Alistar excels at this role, being able to gank effectively as well support his teammates. Below is a rough explanation of how to fulfill this role as Alistar, although one cannot do this perfectly without a good bit of experience.

Flash: Best spell on Alistar. Helps you initiate with Pulverize or escape a tight spot.
Clairvoyance: It is mandatory that at least one person on your team have this spell, as it helps in level 1 jungle fights and can also spot the other jungler, setting them up for counter-jungling.

Other Potential Spells
Ghost: Another good spell with multiple uses: chasing, escaping, ganking, and positioning that Headbutt.
Teleport: Can help you get between lanes quickly; another plausible spell.
Cleanse: Helps you take CC better, but is generally unnecessary.
Clarity: If you really have no restraint and spam your moves constantly, this will help you out early in the game.

Remember to get the improvement mastery for any summoner spell you take if possible.

0/21/9, although 0/9/21 works as well.
Offense: There isn't too much in the offense tree that helps Alistar much, as he really doesn't do much damage.
Defense: Making yourself more durable helps you position better and tank damage better. I recommend going mostly in this tree.
Utility: You can take 21 points in this tree, although I find defense more valuable. Take Good Hands because Perserverance is mathematically bad. Take Awareness and Greed because you won't be getting much gold or experience and you could use more of that stuff.

Unbreakable Will > Triumphant Roar > Pulverize > Headbutt

Get Pulverize and Headbutt at levels 1 and 2 so that you can do your combo. After that you generally don't need to level these as more healing tends to help you more. Always level your ridiculously awesome ultimate.

Greater Mark of Magic Penetration: These are really just placeholders, you can basically use any mark, as your marks don't really help you much. Besides these I would recommend some sort of defensive mark.
Greater Seal of Scaling Mana Regeneration: These help keep your mana up early to late, as Alistar doesn't get too much base mana and I don't really build any (besides Chalice of Harmony). You can also use Greater Seal of Evasion for more durability and Nimbleness procs. Greater Seal of Armor helps reduce minion damage and are especially good for counterjungling.
Greater Glyph of Scaling Cooldown Reduction: CDR is pretty good on Alistar, as he doesn't scale well with much else. Other options for glyphs are Greater Glyph of Scaling Magic Resist or Greater Glyph of Scaling Mana Regeneration for more durability or mana respectively.
Greater Quintessence of Movement Speed: In general, moving faster is going to help out Alistar the most. You can also take Greater Quintessence of Health if you are concerned that you might die early, though Alistar is naturally tanky.

Boots, Sight Ward, Mana Potion: These starting items allow you to get to roaming the fastest which is why I take them.
Philosopher's Stone, Heart of Gold: These two help you get gold income while you are roaming between lanes, while also providing some helpful stats.
Mobility Boots: These boots help you roam the fastest. In general take these.
Catalyst the Protector: Helps keep you healthy and at high mana since you are leveling up later than everyone else. Builds into a Banshee's Veil later.
Aegis of the Legion: A great aura that someone on your team should take; it might as well be you.
Banshee's Veil: One of the best defensive items in the game and it builds from your Catalyst the Protector.
Shurelya's Battlesong: At this point you run out of slots so you start finishing your gold items. This is a great support item, especially good on Alistar.
Randuin's Omen: A formerly great, now decent, tank item that builds from your Heart of Gold. Combined with Shurelya's, you can give your team a large positional advantage.
Rylai's Crystal Scepter: Gives you a little bit of extra damage, a good chunk of health, and a slow on all of your moves (and I think your passive too?). In general you only buy this if the game goes really late since you want to keep a slot open for wards.

Other Good Items:
Mercury's Treads, Ninja Tabi, Boots of Swiftness, Ionian Boots of Lucidity: Different situations may merit buying different boots. In general, Mobility Boots help most while roaming.
Chalice of Harmony: Buy this pretty early if you are broke and are having a lot of mana trouble.
Guardian Angel: A possible alternative for the last slot, especially if someone else got Aegis of the Legion.
Frozen Heart: A great item on Alistar with a good aura and CDR. Sometimes this is an alternative to finishing Randuin's Omen.
Force of Nature: Take this as your last item if you are getting killed by magic damage or especially percentage health based damage ( Amumu, Brand, Kog'Maw, Malzahar, Deathfire Grasp, Madred's Bloodrazor). The movement speed also helps.
Soul Shroud, Zeke's Harbinger, Abyssal Mask, etc.: Grabbing Aura items can definitely help out your team. These are situationally good.
Thornmail, Sunfire Aegis, Warmog's Armor, etc.: Tank items are generally good at Alistar, these just aren't top priority. You can build other items that will help your team more.

Main thing to remember: What items you build is flexible, as well as the order you build them in.

Roaming is in my opinion the most difficult role to fill in League of Legends. While roaming, there are many resources that you have. These are: experience and farm, gold, and location. With each of these comes decisions. You must balance the use of each resource. Sections on each resource are below.

Despite the fact that you are the roamer, you are going to need at least some experience and farm to get you going. You don't want to end up five or more levels behind as you start to become ineffective. Just remember: your teammates need farm and experience more than you. Try to feed off of leftover experience and farm while teammates have to leave the lane. This should help keep you from falling too far back, while still giving your laners optimal farm and experience. In short: don't take farm from teammates but don't let it go to waste either.

The main dilemma with gold is that you aren't getting that much of it. Most of it is getting pumped out of your Philosopher's Stone and Heart of Gold. Your items need to be progressing while also grabbing wards to keep track of the enemy team. In general, the better you are doing, the more wards you can get. Below is a priority list, rating ward locations from most important to least important. Note that this is very flexible based on their ability to do Baron/Dragon, how much the jungler is ganking, etc.

1. Baron/Dragon: These are the two most important neutral monsters in the game. Baron gives a game changing buff, and both give a lot of money. Keep this warded whenever the enemy might be capable of killing them.
NOTE: You don't necessarily have to place the ward where it has vision of Baron or Dragon. You just need to ward so that you know if they are trying to do it.
2. Small Bushes Near Mid: These bushes reveal many different enemy paths, and make mid almost 100% gank proof. These are best for keeping track of inter-lane movement.
3. Small Bushes Near Sides: These bushes tend to be where the jungler ganks side lanes from. These are important wards, but not necessarily as important as the previous ones.
4. Red/Blue Buffs: If your jungler is getting his jungle stolen, help him out! Also, if your jungler is capable of stealing their buffs, help him out! These are generally the lowest priority wards but may be more important depending on the situation.

You can't be everywhere at once, so you have to choose where to go wisely. You can generally help out every lane in some way, but you have to choose which one to go to. Here are your priorities on which lane to go to:

1. A lane is being ganked: If you are close enough to help out a lane that is being ganked, get there quick to support your teammate. This is the most important time to be in a lane. Not only are you trying to save your teammate, but you are also trying to prevent the tower push that will most likely follow.
2. A lane is being zoned: One of the most important roles of a roamer is to keep his teammates farming and leveling at maximum capacity. Depending on the situation, this could be a good time to either gank or support. Gank if you can outnumber the enemy and ensure the kill; support if your teammate is weak and may die during a gank. Do whatever it takes to keep their laner from farming well and to keep your laner farming well.
3. A lane is being pushed: Your towers are important, don't let them fall! In this scenario is another decision between ganking and supporting. Gank if they are overextending, meaning that you are able to kill them by outnumbering or through use of the tower. Support if it is a strong push of multiple enemies and you simply want to keep your tower alive.
4. A laner needs to recall: You can either try to gank the lane and then let the laner recall or just cover the lane. Either way, this is low priority.

Ganking as Alistar is pretty straightforward. Try to run in from behind so that you can Headbutt the enemy towards your teammate. Sometimes Pulverize is necessary to position properly. Try to save Pulverize for an opportune moment, such as keeping the enemy in tower range, keeping the enemy in a teammate's skillshot or AoE, or positioning for a Headbutt.

Knowing when to gank is just as important as knowing how to. Here are some things to consider:
1. Is the person you are trying to gank overextended?
2. Does the person you are trying to gank have an escape mechanism?
3. Does the enemy know you are going to gank?
4. Is the enemy team in position to save their teammate or kill you?

To finish many ganks, you must dive their tower to finish off the enemy. Alistar is a great tower diver as Unbreakable Will makes towers hit you for very little. Here are some things to consider when wondering if you should dive their tower:
1. Is your ultimate off cooldown?
2. Does the enemy have an escape mechanic that they haven't used yet?
3. Can the enemy kill me at the tower?
4. Is the enemy team converging to save their teammate or kill you?

There are many ways to support a teammate in trouble. Being in trouble can range from being ganked to having low health. Here's what you can do:

1. Heal your teammate with Triumphant Roar.
2. Deter and interrupt enemies with your Pulverize.
3. Keep enemies off of your teammate with Headbutt.
4. Draw damage away from your teammate. In general, if you get up in people's faces, they start to attack you. Use your tankiness to your advantage and keep them from attacking your teammate.

This whole guide has basically been in lists of four and this section will be too. In a teamfight, there are many things you can do to help your team win:

1. Come in from the back. This helps you land your Headbutt and also tends to make the enemies focus you. Also, regarding to Headbutt, remember that it can be used defensively as well, keeping an enemy from getting a kill.
2. Once enemies are focusing you, use Unbreakable Will to keep yourself alive.
3. Try to hit as many people as you can with Pulverize, and make sure to get important or squishy people with it such as carries.
4. Keep using your heal; it can make a difference.

5/4/2011:
Switched out Ghost for Clairvoyance in Summoner Spells.
Added Greater Seal of Armor to Rune Build.
Switched out Chalice of Harmony with Catalyst the Protector in Item Purchases.
Switched out Force of Nature with Banshee's Veil in Item Purchases.
Switched out Trinity Force with Rylai's Crystal Scepter in Item Purchases.
Removed Sheen from alternative items.