1. Positioning is key. Before an initiation, you use Q to poke. It hurts and is usually hard to punish...because he made you be quiet. Never initiate, never riftwalk in before CC is used. I prefer to try and target a caster, Q, walk in, E as many targets as possible, walk or R out of most range, Q and E again. Try and prioritize targets though. You should be taking out their most dangerous member. Don't be afraid to R on someones head if they're running away. It actually does damage!
2. Mid is really your best option. I take Ignite every game as Kassadin because it's just too useful. You can even use it to counter potions! Early during laning, use Q to help last hit. Until lvl 3, it doesn't really hurt. Lvl 3 Q hurts though. After you obtain it at 5, land it on the enemy champ every time it's up. At 6, he should be half HP and it should be as simple as, Q, R, E, Ignite, melee 'til dead.
3. Kass needs farm. Using Q to help last hit in the very early levels totally helps this. Also, watch for multiple minions to be below half and try to get a few at a time with E. Using your skills to farm isn't a waste of mana. Just try to keep enough for your combo if you need it. By mid game you should be able to almost completely obliterate a creepwave with E.
I prefer taking Sapphire Crystal/Meki 2x red pot, Tear of Goddess, Boots 1, Sorceror's Shoes, Hard AP item *rabadons or Mejai's*, Glacial Shroud or Abyssal Scepter. Games usually over not too long after that. If it does continue, complete your Archangels Staff and decide on either more defense or more AP. Probably more AP.
9x Mpen Reds
9x mp5/lvl Yellows
9x Scaling MR/Flat CDR Blue's
3x Movespeed Quints
Ignite and Tele/Cleanse