I pulled together all the Skateboard Grabs I knew of, I even got my hands on some No-Grabs too. Ollie airs are clean and look so good.
Grabs and airs are usually done on ramps. Either launching out the top. Done frontside, backside or to fakie back into the ramp.
For most grabs you can pre-grab to get into the air. That means you grab the board while standing on it and do a little hop. Using your momentum and the ramp to propel into the air. Or you can ollie to your grab. Pop your ollie and grab the board during the air.
If you have time and some skill then give the grab a little tweak for style points. Twist and tweak your body to make some of the more advanced grab moves. Pull off one or both of your feet to get more trick combos.
My favourite is the no grab airs. Just ollie high and get into the sky. No hands needed. Time floating through the air with just your feet is the best time.
I am working on a graphic that will show all of the grab spots you can use. Until then look at this diagram by Snowboard Addiction.com a great source for snowboard information. It shows the grab spots on a snowboard. Which is basically the same as for a skateboard.
I find that doing grabs is easier if you practice the grab while rolling on your skateboard. This allows you to square up and stay balanced. At the same time you are finding out how your body feels in each position.
Ask yourself. Can I reach that grab?
Am I flexible enough to make that grab while in the air?
When grabbing a skateboard while in the air. It is better to bring your feet and board up to your hand then to reach down with your hand to your feet. This keeps your body in good form and balance. Not bent over. Being upright in the trick makes it a lot easier to land on your wheels and roll away with balance.
Grabs and airs can be done straight off a launch ramp going as high into the air as you can. The mega ramps they use in the X-games is not far from what you can try at the local skatepark. You might go a little slower but it doesn't feel that way wheel you hit the trick just right.