All of what these guys said- once you get used to it, you'll have it no problem, but there are some things you need to be concentrating on...
by the way, don't worry if you fall, everyone falls their first time.
THE THREE MOST IMPORTANT PARTS OF A SUCCESSFUL RAILSLIDE
1. Speed. Going faster will help you keep your balance- you won't just be sliding slowly, teetering on the rail. Going fast is scary, but you've got to do it- refer to no. 3.
2. Balance/Weight distribution. What happens a lot of times, is you will jump on a rail and will be leaning back. Whoops! Your feet will slide out ahead of you and you will whack your hip or head on the rail. When you jump on, distribute your weight evenly between your feet, and keep them spread about shoulder length. Look down the end of the rail, and if you're going to put your weight anywhere, put in on your front foot.
3. Commitment. The most important part. When you ski up to the rail, no matter how scared you are, you have GOT to make that 90 degree turn or you'll rack yourself, end of story. You got to get sideways and stay sideways. Turn, spread your legs, center your weight, balance, look down the rail, and ride it out. Good luck, bro.
'The road of excess leads to the palace of wisdom.' -Jim Morrison