As is most things in the game, your items will tell you how you should approach your strategy against Isaac. I've had bomb tears, lazers, rapid fire tears, etc. and took a very different approach each time.

General hints:

For Isaac's first form he shoots 12 radial tears. You are safe near the corners of the room. He also shoots retaliatory tears in an arc like the blood tears Monstro shoots, some might go short, some might go over you, so look for the shadows to where to dodge them. I generally just keep spinning in circles around Isaac shooting him and dodge the shadows when necessary. 

His second form he shoots homing purple tears and shoots tears when hit. Again, circle around him, dodging tears when necessary.

When he starts spawning the angel babies (3rd form) that's when I usually pull out my consumables. Use your death card, my little unicorn, bombs, etc. Use em if you got em!

Wiki link:
[Isaac (Boss)][1]

  [1]: http://bindingofisaac.wikia.com/wiki/Isaac_%28boss%29