Not Updated For Current Season
This is my first guide, I figured I'd post it since it's similar to Phreak's as well as a couple others I've read with a few minor tweaks. My last game I went 17/2/15 on summoner's rift going mid against Heimerdinger.
The basic strategy in a team fight is to open up with Hemoplague, followed by Tides of Blood, then hit your target with Transfusion/Ignite and use Sanguine Pool to slow everyone and become untargetable. With a little practice you can blow up people end game with the greatest of ease. Try to keep Tides stacked to 4 during fights and always always use Transfusion as soon as it's off cooldown.
You'll want to start out with your Doran's Shield and a Health Pot. Once you're about lvl6 you should have enough money to pick up your Spirit Visage, if not then grab the Rejuv Beads and Mag Resist cape that make up the Spirit Visage and your Boots of Speed and head back to mid. From here just keep pressure on mid with Transfusion and play smart. If you can get a level or two ahead of your opponent then it's pretty much over as long as you watch for ganks. Once you have your spirit visage start working on your Rylai's, and follow it up with Mejai's and Leviathan in whichever order you prefer. If you don't like the stacking items you can replace these with a Void Staff and/or Sunfire Capes, but I like how OP you get with 20 stacks of each.
Again this is my first guide and I've had some wild success with this build. Please rate and comment, and good luck on the battlefield.