This is a guide that I've made as to how to play Swain the Master Tactician. He is by far my favourite champion to play on League, and is a mighty fine bruiser and tank the way I play him.
Hopefully it works out for you, These next few chapters will tell you how to dominate with him, and hopefully win lots of games :D

The runes I take with swain are as follows:

Flat Magic Pen Marks x9
Flat Ability Power Seals x9
Flat Ability Power Glyphs x9
Magic Pen Quintessences x2
Ability Power Quintessence x1

These runes allow you to start off with a large amount of Ability Power at the beginning of the game, if you start off with the recommended item build, and it allows you to do a fair bit of damage initially to enemies.
It will give you:

+19 Ability power to start off with
+12 Magic Pen to get through their initial magic resistances

The Masteries I take as Swain are as follows:
21/0/9

In detail, I take in the offense tree:
1 in Summoners wrath: To increase AP while your ignite is on CD
4 in Mental Force: For 4+ more AP
4 in Sorcery: For 4% CD to start with
1 in Arcane Knowledge: To have 10% magic pen to begin with plus your runes
3 in Havoc: Increase damage dealt by 1.5%
4 in Blast: 1+ AP per level (+18 AP at level 18)
3 in Archmage: Increase AP by 3.75%
1 in Executioner: Increase damage done by 6% to targets below 40% health


In the Utility tree I take:
3 in Good Hands: reduce time dead by 10%
3 in Expanded Mind: +12 mana per level (+216 mana at level 18)
2 in Meditation: 2+ mana regen every 5 seconds
1 in Runic Affinity: Increase duration of buffs by 20%


These masteries allow great utility and burst damage at low level, and tanky and durable Swain late game.

The item list I build as swain is the same as it has been since I started playing him in January 2011.
The items are in order of importance:
Dorans ring
Rod of Ages
Rylais Crystal Scepter

Then from these 3 items, you can expand into whatever items suit your game, whether it be:
Void Staff for magic penetration against tanks or teams with high magic resist.
Abyssal Scepter for teams with a bit more AP then usual, giving you AP and magic resist.
Mejais Soulstealer for additional AP on kills/assist (I highly recommend getting this item only once you have at least 5 or 6 kills, and 0 to 1 deaths).
Rabbadons Deathcap for increased AP damage
Zhonyas Hourglass for teams with heavy AD and 2 second invulnerability plus AP.

You will notice in my build, that there is no Arch Angels for mana regen, and I will explain this in a later chapter.

The skill sequence for Swain is a very hard one to master, as well as to time and place against certain enemies.

His particular abilities don't allow him to farm as easily until he gets enough sustained mana to toggle his Ultimate on and off.

When engaging in a fight or team fight in general, I tend to be the initiator with a well placed Nevermove. This will snare # targets in place for the team to engage on, at this time you will want to cast Torment, Decripify and then activate your ultimate and chase them down. This would also be a good time to activate Ignite, because Ignite ticks with Torment increases damage output due to Torments passive of increased damage done by Swain by #%.


Early game:
Usually all the time, I lane Swain middle against any and all champions.
His initial damage when he hits level 3 can succumb targets to death in a matter of seconds if played right.
I play Swain in a very aggressive play style, this can lead to my downfall against certain champions (Mainly Cassiopeia and Leblanc) but it does get advantages if you get an enemy that thinks that Swain isn't a good champion because you never see a good Swain. (Key tip: Never. Underestimate. Swain.)

Mid game:
Around level 7-8 I branch out and if need be, I will gank other lanes to assist them and also to get gold. This is the time of the match when Blue buff takes priority of the jungle for you and will be for the rest of the game.
My play style relies extremely heavily on Blue Buff. It has large advantages and large dis-advantages, lets point these out in detail:

Advantages:
Consistent Mana regen
High Cooldown Reduction
Sustainable in lane, and can deal lots of damage continuously

Dis-Advantages:
No mana regen
No cooldown reduction
And need to still last hit, or heavily conserve mana for fights that look like you can win.
(When you don't have Blue Buff, never walk around with your Ultimate toggled on, it drains your mana at 25 per second and is very mana expensive, but if your in a team fight, your Ultimate is essential to survival and kills, so if you run out of mana, still try and contribute with basic attacks and whatever little mana you can spare).

Here is an example of great and very well played Skill sequences used as Swain. It made the entire enemy team rage and call for a nerf on Swain. He doesn't need a nerf, I just played well :D

25/0 Swain

The summoner spells I take with Swain are simply Flash and Ignite.

These spells give Swain great maneuverability and extra damage mixed with Torment.

Flash is used to either Initiate or get out of tight spots, so use it carefully as always.

Ignite may be used to stop healing effects, or to terminate ones life with fire :3
It is very useful at low health, so preserve it when needed.

This is the summary of the complete guide, and this is how I play Swain the Master Tactician.
This guide may be a bit hard for people to grasp due to its high risk availability to always have Blue buff but because I play in teams of premade 5's, Blue is always reserved for me. Ask your team jungler for Blue around level 7-8 otherwise, go for it yourself.
Ever an opportunity to steal the enemy blue? Do it.

Appreciate your time reading my guide.

"The early bird guts the worm". - Swain