The reason people usually get off axis while trying to grab their spins is they are going for their grab before they set their spin. (or they are scared and spinning early.)
Working up to it:
If you have a place to practice, such as a trampoline, etc. that will help you the most. Work on them till you can do them in your sleep, using the steps below. (If you don't have access to a trampoline, don't worry, you can still get it.) On snow, practice spinning tucked up 3's. Try to get them both with your skis parallel and crossed up. Once you have those down, then you can add your grab.
Try these steps:
1)Pop and set your spin a little harder than you would a normal spin, keeping your eyes on the horizen. (Once you get them solid you can look wherever you want, but for starters, keeping your eyes level is important to keeping you on axis.) Also, make sure you wait for your pop and don't spin before you leave the lip. Unwind your upper body right as you pop off the lip.
2)Once you're in the air, tuck up and go for the grab. Think of bringing the skis up to you instead of bending over to grab your skis.
3)Try to stay tight in the air, hold your grab as long as feels comfortable, and then spot your landing like a normal 3 or 5.
4)Once you get this you can start working on style. Think about staying tight in the air. Picture the position you want to be in if someone was going to take a pic of you, then concentrate on that while you're in the air. It helps to practice your grabs without a spin so you can get the feel of the position you will be in, then try to add it into your spin.