Here's my build for Swain, and how I like to play him.
Keep in mind, that this is also my first guide, and I don't usually look at the other guides..
During this build you will notice I chose Clarity, and Mana regen.. I will explain why in a chapter down below.

First off is runes. I use;
9 Marks of Magic penetration. (Every caster needs magic penetration)
9 Seals of Mana regeneration. (Swain requires alot of mana regen if your going to spam his abilities)
9 Glyphs of Abilitypower. (The extra dmg is always a welcome sight, i use flat for increase dmg at start)

As for quintessences i use 2 flat ability power and 1 magic penetration.
Giving me;
+ 10 Magic penetration
+ 19 Ability Power
+ 3.7 Mana Regen/5

Torment is the main dmg skill that swain has, and you will use it often to harrass enemy players, and also help get the kill.

Nevermore gets upped last, as the best use for it is to keep an enemy rooted until you can get close enough to use your other skills. But putting skillpoints in nevermore does not increase the root-time. Altho' it is wonderful for farming casterminions at lvl 4 and 5, the other skills does more dmg to single enemy champions.

Decrepify is the secondary damage skill, which like Torment, also deals Damage Over Time, but has a range, where the enemy champion can walk out of to end the DOT.

The skill sequence when attacking or damaging enemy champions should something like this.
"Run towards them, cutting their escapeway. Hit them with Torment, Decrepify, Lock them in place with Nevermore, Hit them with Ignite, activate your ulti(Ravenous Flock). Then follow them, while constantly blasting your abilites as the CD allows. If doing this vs a tough champion that can take alot of magic dmg, just use Clarity to hold out the combo abit longer. This will kill most champs within seconds while leaving Swain almost unharmed. (Maybe a little low on mana afterwards. But with the Golembuff(Blue Buff), The mana will be no issue.)

The Sorc boots are selfexplanatory, all casters should have/get it.
The Rylai's Scepter has great synergy with your ultimate.
The Deathcap is also selfexplanatory, all caster should have/get it.
The Archangels Staff is great for it's manaregen + added manapool.
The Frozen Heart is wonderful for the debuff and armor vs the regular fed Yi or other Melee champs that will focus you. (Which trust me, happens alot.)
The Force of Nature is pretty sweet on Swain. Why? Because the passive health regen is a nice add-on to swains ulti. The magic resist is sweet. (Swain is a caster-tank if you didn't know..) And the added movement speed is also pretty nice.

If you are facing heavy casters on the other team, like Veigar (Anti-mage...mage..) or Kassadin (Mage-assasin). Then switching the Archangels Staff for Abyssal Scepter is also workable.
Or when facing Karthus, switching the Force of Nature for the Hourglass is usually a good idea.

I chose clarity and ignite. Ignite is a must, if you want to dominate your lane.
Maybe of you might think of Clarity as worthless on swain, as you get mana back from killing minions. Well good sir/madam, are you then telling us that you _never_ run out of mana with swain? Either in teamfights, or vs a enemy champion that refuses to let you lasthit minions?
I just did a 1v1 with another Swain that had flash and ignite. (Also a nice combo.)
But since we both did our best to harrass each other, he went out of mana the same time as me, while i popped clarity, and stood between him and his minions constantly, while farming freely, and harrasing him.
His flash didn't help him much then....
Little bit later that game, i also killed him in mid lane, after he went to heal twice. (again, thx to my clarity.)

PRO
+ Very strong and durable vs all champions. (Except strong casters)
+ Does alot of damage, and played correctly, should be able to handle going 1v2 anytime. (Unless it is 2 heavy bursters)
+ Dominates the lane pretty easy after lvl 6.
+ Unless focused by at least 3 ppl in teamfights, you virtually untouchable and able to control most teamfights and outcome.

CONS
- Very Golem Buff dependant.
- Not able to easily escape ganks.