My "trick" for back swaps (at least while learning them) is to have your right hand (assuming you slide right foot forward) dropped back slightly behind you before your get on the rail (so on the approach...kinda like you're gonna 270 on, but not quite as far) and then when you get on the rail continue bringing your arm around and just as it crosses infront of you jump up and complete the turn, this will keep you centered over the rail. Using a front toe dip scissor (like for a back 270 out) also helps, combining these two things together can lead to a smooth, more controlled back swap which will help keep you on the rail and can help with combos after you get used to them (since you can do them sooner you can set your edge easier for combos). Finally, practice sliding opposite foot forward....it can be a HUGE help.