It also helps to hit a little jump switch lots of times, and just get the feel of popping off switch and doing a smooth, balanced 180. That will really help getting the 90 onto the rail. You might also want to try hitting rail that's got a bit of a jump in front of it, so you've got more time in the air to get balanced. When I go in fakie I like to hit it with a lot of speed, drop my tips as I rotate the 90, and then even things out when I hit the rail.
--Disgruntled and Nostalgic Crazed Posting Bro!--