Your potion's at the beginning can be anything but i like Health because you get plenty of mana from your passive by killing creep. Also on your Runes they can be anything but I've had better luck with AP runes than anything and i like the late game runes because Swain alrdy get OP late game so being that much better makes Swain one of the best champions in the game. The skill sequence is 100% the best i've found in the times I've played it alows for massive dot( damage over time) damage with helps alot if someone runs to the tower when you hit them. On the mastery's I think this is the best set of mastery's you can get it alows for a great game early and late.
When i gank someone I like to cast Nevermove than ult on them once your close than hit Torment than hit Decrepify to slow them while they're running and justauto attack them and let your ult do the work and they stand no chance.
Farming creep i easy on Swain. If your in a group of minions that has 15 or more ult on them and you will gain HP and mana just remember to cancel your ult before you start to lose mana.