There's a relatively specific list of items an owner/pilot can do as preventive maintenance with no A&P involvement (lights, brakes, tires, replacing seat belts, fluid replenishment, oil changes, etc.). Outside that list, you can basically do whatever you are capable of provided that your A&P (or IA, as appropriate) inspects the work, signs off and returns the plane to service. Some mechanical ability, a willingness to research and learn, and an A&P that will do owner-assisted maintenance are, IMHO, key factors to keeping ownership affordable.
Certain items, primarily those involving the static system and the transponder, must be done by a repair station with the appropriate ratings. But the vast majority of the work can be done by an owner with a cooperative A&P.
