How to get the hang of 270's on rails.

start out on low, wide rails or boxes. once you have them down you get em pretty much every time

Practice 360s off tiny jumps (side booters just off rail are a start), practice sliding with the foot that will land forward - for example, if you spin left, make sure your fairly comfortable with left foot forward slides. Find a mellow, low flat rail. They can become pretty consistent with a lot of practice, but they do take a lot of work to get that way.

BWalmerFind a mellow, low flat rail.

or box...forgot that.

Find a butter box and do 180s on. If you can't do 180s on then you probably shouldn't be trying 270s. Just work your way up from there. It helps if you happen to grind naturally the way you land. If not then learn to grind unnatural first. Urban 270s on are more difficult IMO then straight on 270s. Progress your way up to skinnier boxes and then the same with rails.

BWalmermake sure your fairly comfortable with left foot forward slides.

I can't hit a rail left foot forward without slipping out most of the time unless I do a 270 on, I know I'm not the only one. I guess it goes to show what practice can do... get a pvc rail, it helps a ton.

Get control on small jumps, you have to spin fast and have a good pop, make sure your whole body spins at the same time
Then try it on a low flat box, make a good pop and make sure to land with feets wide appart, try to enter the box from the side, not with the box between your legs
To get used to the feeling of a rail the best alternative is Pvc pipe, set it lower than the kicker at first but put it higher as you get better, this will practive you to get the tips of your skis over the rail before spining
once your good with this you can try to do down rails and boxes
Once you get it right it should be easy every time, muscle memory does the work

I find tubes to be the easiest to learn on. the main thing like other people said is sliding unnaturally so you have to remember to transfer your weight to your feet that will be in front.