keep and really wide stance coming into the rail (lol) and keep your feet pretty far apart once your on the rail. also keep your eyes on the end of the rail and that will help with not sliding off the side of it half way down
i would recommend you learn you to slide boxes first but if you can do that then the next step up is rails -- the basics are approach the rail with it between your legs, pop, but as you pop start putting your wieght down the rail, putting my arms down the rail and looking at the end helps me do this. land on the rail look down it at the end and COMMIT
down rails with street setups are a lot harder .. come at it from the side pop harder.. you got to make sure you pop high enough or else you'll get owned...with the street and down rails it is really important to get your weight down the rail as those are the rails that are easy to slip out on