You are approaching your favorite jump of he day and feeling confident in your skills. You stuck your last three tricks (two of which were tail-grabs), so your hand knows where the tail is. Now what you want to do is a backside three with a grab. You stop at the top of the runway and imagine yourself doing it.
Not only do you imagine it in your mind, you go through the motions with your body: you think about waiting until the very top of the jump before you start spinning. You think about how you will turn your shoulders the opposite direction of the spin just before spinning, and then turn them hard in the direction of the spin as you go off the jump. Try those motions a few times while standing. Now you will consider the grab–your hand knows where the tail of your board is without having to look, so that part should come easily after a few tries. If not, forget the grab until you have 360s dialed in, and then come back to it.