You're probably letting your momentum take you too far because it's uncomfortable when learning blind swaps. Look at the end of the rail, be completely horizontal on the rail, grip, spin, and practice until you find something that works. Shitty advice probably, but whatever
"If you're getting rad, and there's no one there to see it, are you REALLY getting rad?"
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.
or you can keep doing what you're doing but just sit back and get it blunted, that's what I did when I learned it, but I learned how to do it at the end of this past season(literally the last day). So I haven't gotten to experiment with it yet, but what those other guys are telling you to do sounds good.
You're probably letting your spin on the switch-up carry you off the side of the rail. Think about completing that 180-degree spin while still traveling in the same line down the rail. You might have to alter the way that you set your swap so it doesn't set you to one side or the other.
Assuming you know what a switch up is...
doing a switch up spinning back towards the beginning of the rail. If you were to stand after the rail and watch someone doing a back switch up you would see their back first, opposed to a front switch up where you'd see their torso.
I told her I didn't want to do the project, and she said that she would do the entire thing if i railed her up the ass, smackers and all. ~Midget.