the key is looking at the end of the rail. I wasn't doing that for the longest time and I kept slippen out and landing on my ass. looking at the end keeps you centred and also keeps your weight in the right place. I also found it helped when I was learning to have one ski on either side of the rail when APPROACHING the rail. So the rail is kind of in between your skis and if you kept on going you'd be hurting. This helps ya out when landing on the rail, I found, because it sets ya up in a nice poisition. Also, if you leave the ground for your 90 degree rotation with your shoulder width apart, you'll land o nthe rail that way. Don't leave the ground with your feet together.
Hope this helps.
Justin the Hick