I can't find diamonds in minecraft. What is the easiest way to find them other than level 12?

The easiest way is to look around places that have water and lava in that area. Where lava and water are in the same cave or area then you know diamonds are around. The method I use is digging on big patches of roses, you find diamonds this way as well.

That is the easiest way. I like using the "mine shaft" method, but it still needs to be done below level 16.