I have been lucky enough to do some air to air over the last few years. One thing that is true, is that every flight teaches you a huge amount. I am not an expert or a pro, simply a lucky sod, but I offer you some things I have learned.
Formation trained pilots are the ideal. You really want to be flying with folks who are happy and skilled in formation. As the gentleman said above, a flight briefing is essential. Plan out what you are going to do, where you are going to do it and how far apart you intend to fly.
Your choice of camera gear relates to this; a lot of the pros use relatively wide angle lenses (50-120mm) an get the aircraft pretty close together. This obviously requires experienced pilots and calm air. If you're just flying with a friend from point A to point B, you might get some photos en route, but you don't want to fly so close together. A longer lens will allow some distance between you. A 70-200mm lens is great. Longer than this and you'll run into issues of being able to point it out of the aircraft and keep your head inside! You can plan for the desired separation on the ground by standing near the subject aircraft with your camera and seeing how far back you need to be for your lens focal length(s).
Open cockpit aircraft are great to shoot from, but often are biplanes with wing struts and wires that get in the way. A T-6 or Chipmunk with a sliding canopy is pretty cool. Opening windows in the cockpit can work well if they're in the right place. Don't hold your camera in the slipstream, you cannot hold it steady against a 100+kt wind. Perspex is usable but subject to reflections. If you have to shoot through perspex then wear dark clothing and use a rubber lens hood so you don't scratch the canopy. If you have a removable door (Bonanzas do) then shoot through the door; it's fabulous, but do use a harness! And make sure you secure the harness to a solid mount point. A seat will not do.
The ultimate best place to shoot from is the back end of a B-25 with the tailcone removed
That sort of thing allows you to get those lovely head-on shots, although it is expensive. However most GA aircraft will allow for great side and 3/4 shots.
Calm air and lighting conditions are best at sunrise and sunset. If you can plan a shoot around these times, you'll get some great pictures, and the calm air means you won't get jiggled about too much. This also means you can lower the shutter speed to get nice blurred props - you want to shoot at 1/250 or less to achive this; preferably around 1/100 although this does take practise, so take some faster shots first to make sure you have some in the bag. You should be flying at the same speed as the subject aircraft, so motion blur comes less from the aircraft speed (like if you were shooting a fats moving plane at an airshow) and more from the vibrations in the immediate environment.
If the purpose of the flight is just for a photoshoot, then find an area that is not too populated so you don't annoy too many folks, and this should also give you cleaner backgrounds (think forests or lakes). Fly a racetrack pattern in this area for a while to take advantage of the changing angle of light.
Good communication aloft is essential. The subject aircraft should fly on the photo ship and never take their eyes off that lead. The photographer can use hand signals to the subject pilot to move him around, however these signals should be worked out in the pre-flight briefing and then adhered to, to avoid misunderstandings.
Also brief for a lost-sight event; if one of you loses sight of the other for any reason, you need to have a procedure in place for who flies where to regain sight. Likewise if some other eventuality occurs (bird strike, engine failure etc), be prepared for whichever aircraft is involved to break off.
I hope this is useful info...
Have fun :-) Nothing beats the buzz of a successful air to air shoot!
You might also think about joining ISAP , the International Society of Aviation Photographers. Great bunch of folks.