Make sure you have back 180s down solid so you can get used to turning your fingers in that direction. Then just practice on getting your kickflips turned slightly backside and keep turning them more and more until you can at least get a 90 degree kickflip. then you can just pivot/slide the rest of the way if you have to (sort of how I do mine).
Also remember that when doing this trick your middle finger will be curling in while your index finger will stay straight. It will be your middle finger that will be doing all the work in getting the board to pivot..
edit: Just saw the trick tip by Taylor that was posted and I basically said what he said. Just watch that video haha.