I had a lot of trouble sliding rails, but i finally got it. I slide with my left foot forward naturally, and all of last year i would slide off to the right side of the rail. I was really getting pissed, cause i would practice and practice, so finally at camp this summer i tried starting from the left side of the rail, and it really helped me. So if you are sliding off one side, try starting from the opposite side a little more
jibba jabba