Well, I can give you only one advice: Go out and train! (this usually works for Parkour)
For SA it would be: Find a plane and train!
Best planes would probably be Hydra and a Stuntplane later, when you accquire more skill, sine Stuntplane is much more sensitive, so it needs a steady hand. Try flying under various objects like bridges or through tunnels (later). As you make progress, you might wanna try to do all those stunts again, but this time upside-down.
That's pretty much I can say to you. Watch various vids (like Ghostchilds "Wichita" or "Pokker"). I've also made a vid "Fly Till Die". I've posted a link in SA stunting forum, might give you an inspiration.
As for car stunting, all I can say to you is that the best cars would be Bullet and Hotring Racer.