So alistar is a tanky support with some overpower abilitys, they might not doing damage but they can kill allright! (had a triple kill the other day 3v1 in a turret ;) ). Alistar can be played as a TANK in case of no tank on top (such as teemo) witch it would be a very intresting way to play since he got the ulty with 50% reduced damage. He is a meele champion but a very fun to play. His counters are Jana, Lulu and cho-gath but he is good against Vayne, Tristana, Blitzcrank, katarina, leona, thresh and ezreal!
Make sure you play a bit aggressive when the adc you supporting is playing aggressive too and always get in front as a tankier champion than the adc.

alistar hase a problem with his mana so i take clarity to solve it. but if you don't play good alistar i suggest you instead of clarity to take flash. there is a combo you can do with it. let's say you have a teamfight over the second turret (no matter if it's yours or theirs) you go to jungle and stick alistar on the wall close to the turret where the enemy will be. Flash in the turret, ulty so that you take very little damage q them all if you can (or your target at least), then w the carry to throw it to your team and take it out, if the adc flash again back then use exaust and heal yourself for the passive and try to escape from everyone in hopes that your team will catch up to the enemy adc.

Alistar makes a great Teamwork with Blitzcrank (in case of trollpick or just a fun game)
(example) http://www.youtube.com/watch?v=Unv7lXo2xR8
he can combo with nidalee. the best adc you can have on your side is vayne or quiin and you can combo with both of them (vayne stuns on the wall and then you stun again in case the enemy didn't die yet)
Alistar in general can team up almost with every champion and is about for every situration. For me he is the best (and most overpower) support in this game.

in the early game you want to have a hard lane (that is what alistar does). So it's ok if you play a bit aggresive.
Always tank the turrets when no minions left! Do not afraid! if you are 3v4 (you are the 3) in front of a half life turret and there are no minions left then ulty and tank the turret do not afraid of damage and even if you die it doesn't mater cause you need to brake as much turrets as you can. if you have about 5 minions to tank the turret but again 3v4 then tank the champions, go in front of the turret and hit it, they will focus you but you will take very small damage so they will come closer to hit the other champs. in this case never hit an enemy champ sto that the turret won't focus you. if an enemy turret try to kill one of your team mates drive him away fast so that he don't manage it and leave the turret for the next comeback.
in a teamfight always focus (in damage) but weaker (in life and armor) <that means the adc or the ap> get in the enemy line and drive thet champ to your team so that they take it quick and make the game easyer.
In general Alistar is the person who will take the most damage and might die but will eventully win the game if he play right.
try to take us much assists as you can so that you reach your full build (in an good game that is played about 60 min you might not reach your full build cause alistar is not farming, if you farm then you might reach it but it won't be nessesary)
on a 3 v 1 (you are the one) stay in turret and take triplekill (hard to do but i will explain) they will dive, q them all so that the one taking the damage from the turret have a low life and the others won't hit you for a bit. Ulty fast! (if you have not ulty that means that the enemy team is stupid to dive before 6th lvl -_- they will die in an instand...) never use your w to drive them away, only to drive them in! so with the q on of them dies (if not exaust him so that he loses as much life as you can and the mid will kill him) if the other two continue hitting you (bad for them wasting their ammo) go in front of the one and hit him until he looses very much life from the turret, he will try to escape and then w that champ to drive him back in the turrets range and take the kill (i know it's easyer with blitz... whatever) now you have left with only one person, and here comes your adc (or the mid, or both) to finish them off and if you live by then ... gj :D (guarandeed you will live)

Alistar can interrupt lot's of things, drive away or closer champions and do much more!
all you want to do is to be ahead of the enemy champion, that is why you use mobility boots with the furor or the green enchantment. in case of blitzcrank you always be in front of your adc. you don't care if he pulls you.
In case of nidalle avoid her spear but take it from close range if it goes for the adc
in case of lux try to stop her ulty
in case of catarina stop her ulty
in case of jarvan throw jarvan of his space to save your teammate (WARNING! don't drive away champions with low life, you save them! you don't want that!)
in case of fizz drive him away ONLY after his tricks
in case of catelin take the ulty, if you have low life do not afraid to use your ulty, if you have 51% reduced damage, your ulty will give you another 50% and you will have 101% reduced damage... that makes you so much tank ...
in case of varus ... just avoid
in case of jana jana can counter alistar with many ways as long as nami sometimes, whatever the situration is if your cary hase low life, just drive the enemy adc out of range.
in case of carthus ... you are doomed
in case of fiddlesticks well easy... drive him away ;) you can cut his lifedrain and drive his ulty for a little away from your team. if he uses hourglass he is a noob ;) hourglass he must use only if he have very low life, his ulty must follow the team
in case of talon ... you are tank, he is ad ... he can't do much to you
in case of zed same as above
in case of garen ... man that is tough ... his abilitys are unbreakable but still you can drive him away
in case of shen ^ same us above. i never tryed to break shens ulty with my w so i don't know if that is plausible ...
in case of ashe, take her ulty if you can. if you can't drive her away and ping fast where the ulty goes with the denger idicator
in case of graves, keep a distance but play aggresive
in case of ryze... good luck with that...
in case of taric well let's se who is tankier... he takes big amounts of ad with his ulty but you take big amounts of armor with yours. so you are his physical enemy and for me his physical antipick.
in case of aatrox. check if his passive has been used, if yes drive him close to your adc and q him so that you kill him, if no then drive him away.
in case of lissandra, watch out her w
in case of zac ... well .. nothing ...
in case of quiin ... FAST DRIVE HER OUT
thresh : let him pull you .. he will regret it ;)
in case of vi, drive her of after her ulty
in case of elise same as above
in case of khazix drive him off
in case of syndra, avoid everything, she won't come close to you
in case of rengar, red wards everywhere, oracles and you are ok
in case of diana, drive her off
in case of zyra avoid her stun and her summons
in case of jayce avoid in ranged form but be close in mele form
in case of draven, drive him away from hes axes ... he will rage quit ;)
in case of darius ... well it's darius ...
in case of hecarim, exaust q and w might save your adc
in case of lulu ... ruuuuuun
in case of fiora, she hase unbreakable ulty and a good q ... dud she is your antipick
in case of nautilus drive him off
in case of ziggs drive him close to you, he plays reanged game
in case of sejuani w her when she w you ;)
in case of ahri, on her e get in front and then drive her off
in case of volibear drive him off
in case of syvana same as above
in case of xerath he will never come close to you
in case of riven, exaust and drive off or in
in case of wukong, drive him off, use oracles and red wards
in case of leona, drive her off the adc
in case of yoric ... no data...
in case of oriana, she will never come close to you
in case of rumble ... FIREEEEEEEEEEEEE
in case of brand same as above 2
in case of nocturne q and w when he hits someone else
in case of maokai drive him off
in case of karma ... no much to do
in case of renenkton same as above
in case of catelin, combo her
in case of trundle, drive him off
in case of irellia, drive her off
in case of leblank, same as above
in case of akali same as above
in case of urgot same as above
in case of swain ... dooomed
in case of kennen same as above
in case of malphite combe on his ulty ;) same results, less pain
in case of malzahar ... run
in case of shen run him off
in case of cog maw ... no much to do
in case of mordekizer drive him off
in case of ezreal, try to make him miss
in case of udir, drive him off
in case of nasus same as above
in case of gragas same as above
in case of pantheon same as above
in case of vladimir same as above
in case of heimendigger wards and same as above
in case of anivia drive her off
in case of miss fortune same as above
in case of olaf same as above
in case of zilean ... run
in case of morgana same us above
in case of jax stay in turret
in case of tristana ... eat her
in case of sion run (drive him off)
in case of singed run in front of him, if he pulls you back w him away and run opposite
in case of eveline drive her off
in case of twisted fate, wards and be prepared
in case of kassadin drive him off after his ulty and run
in case of gatplank drive him off
in case of corki drive him off and avoid ulty, you don't want to make his ulty too strong ...
in case of xin drive him off after his e
in case of sona drive her in
in case of dr mundo drive him off
in case of galio same as above
in case of twich same as above
in case of chogath same as above
in case of amumu drive him off his ulty
in case of shaco drive him off
in case of rammus dreive him off