im not that good at them either, but this is how i do it. if your not ballenced on the rail then dont even try it, you have to be thinking about the 270 like the whole time your on the rail. if you get on rightside since how you turn to get on the rail is the opposite of how you are gonna 270 off, you cant still be rotating at all when you are ready to spin off. then when you get on the rail have your arms and body slightly twisted toward the direction your gonna spin, then at about the end of the rail, i try to pop alittle harder off with my front foot, and while your poping spin your whole body alittle more but not all the way, then right after you pop off try to spin your whole body, especially try to spin with your hips, cause for me i sometimes get only my upper body twisted and my legs are behind, then tuck your legs up and hope you spun enough.
so its all about pretty much starting your rotation one you get on the rail.
and it also helps to bring your back arm across your body and tuck your arms in.