Start small. Get really good at sliding rails and doing straight airs before doing tricks. Get really comfortable at the tricks you're doing before progressing, though I think with rail tricks it's good to mix it up so you don't get too used to doing one trick, thus making doing others more difficult. Watch videos, picture what you want to do in your head, then fully commit to it when you do it. And above all, ski with someone better than you, nothing will make you progress more than this.
Also, if you really want to progress, go skiing as often as possible--it's really really fun, and if you're not skiing, you're obviously not getting better.