You will be amazed at how much a Polarizer helps bring out the sky color. If you don't have a polarizer, at least make sure you're using a lens hood. If you don't have a lens hood, I posted a link to a UK site with paper lens hoods you can print/cut yourself. It's better than lens flare in your images.
Also, on propeller aircraft, you'll want to switch to Tv (shutter priority) and shoot at 1/160th, 1/200th. 1/250th is about the limit. At 1/320th you will get too much "stop", not enough motion.
You might not think of this... but shots of the aircraft approaching are far more interesting than shots of the aircraft departing, unless it's a jet with afterburners going, I do my best to get the shot of the aircraft approaching. With that in mind, finding the best spot to shoot at an airshow is important. Also, you should account for the position of the sun.
Oh... and if you normally turn all of your AF points off except for the center, turn them all back on when the aircraft are in the air. You won't "lose" an aircraft so easy.
Don't forget to bring something wide angle for static shots. Frankly, I think it's worth having a 2nd body with a wide angle lens. For some, that might be asking too much, but I think an airshow is like a baseball game. You have some stuff going on at a distance, and some stuff going on close up.
Good luck.
--
'In 1983, the game of golf had a firm grip on the waist of my boxers and was administering the death wedgie. I had a dose of the atomic yips and after missing 10 of 11 cuts by a single shot, I was ready to quit and apply for a job as a wringer-outer for a one-armed window cleaner.'