Doing backsides is almost as easy as doing frontsides. The one catch is that you are landing with your back to the rail so it is much more difficult to see where you are landing. (and to land properly) Obviously you will need to practice backside stalls on curbs and planters before you work up to rails and slider bars.. And after practicing there for a while your body should learn how to twist and land correctly, and so rail slides will come naturally for you.
Assuming you already can do a backside stall, the slides will be easy to do. First, you need to find a low and not-too-steep bar to practice sliding on. Just about anything will do. Once you are ready, start out by approaching the bar slowly, and at a fairly slight angle. As you near the front of the bar, jump high, and pull your legs up. Keep an eye on your landing spot as you rotate through the 90 degrees and as you are coming back down, unwind and drop your legs onto the bar. The only hard part in doing backsides is getting your legs onto the bar properly. (Duh!) Always watch where you are landing, otherwise you will fall. Also be aware that you may have to jump higher and harder to get on properly. Once you are on, your legs should be about shoulder width apart or so, and you should be grinding in the space between your middle two wheels. As with any trick, the key to having a successful backside in your bag of moves is practice. Just keep at it, and dont expect to learn it in 10 minutes, a hour, or maybe even a week.
So to sum up, concentrate on your landing, your stance, and commiting to the rail. Just as in frontsides, once you are on you just glide it out-- but getting on is the hard part! And to reiterate the commitment issue, if you go up to a rail and only give it 50%, you will fall and possibly get hurt. Trust your skills, and know that you are giving it your all-- but even then you still may fall. Once you get over this mental block, it will all start coming to you..