a 270 off is actually really easy once you get the feeling right. practise sliding the rail with your front edge running on the edge of the rail. ie. the back ski should be flat but the front ski should be digging into the "heel side" of the rail....if that makes any sense. that motion will stop you rotating further around than you need to. eg. if you slide left foot forwards like i do your left ski digs into the rail to stop you rotating further to the right (end eventually to a switch position).
if you're doing it on a box (assuming you're sliding left foot first) make sure you try to land on the left side of the box. sort of imagine the other side of it isnt there. its the side of the box you need to get the rotation.
when you get to the end of the rail/box with your front ski setting the rotation you can pop up and spin off it. at first i popped more with my back ski and used the front one just to set the rotation but it becomes more natural over time and the feeling of riding out switch gets better and better.
i suppose the main thing is to get used to sliding the rail/box with the edge digging in rather than just on two flat bases...if that helps..