First clean your return air coils carefully with a stiff brush and a vacuum. After that flush them out with a foaming coil cleaner. I like the stuff you can get at HD. Works well and has a pleasant, clean smell to it.
If a lot of the fins on the coils are bent over and blocking air flow use a fin comb to straighten them out. Then use good washable filters. The best, IMO, are the ones that are made out of a stiff black plastic woven material and are thin enough to to slide down properly into the built in filter tracks that most good A/C units will have on the sides of the front of the coil fin surface. Some of these filters even come with heat sealed edges which make it even easier to slide the filter into the track.
Depending on how you use your A/C these should be cleaned monthly. When you clean them add pan tabs to the condensation tray to keep it clean and less likely to block the drain line.
Fin rake: DiversiTech T-101 Fin-Tool Nylon Kit, Fits 13-20 Fins Per In.
Coil cleaner: A/C Safe AC-920 Air Conditioner Safe Foam Coil Cleaner, Cleans Coils Q
The cap that comes with this makes a pretty good fin comb.
Pan tabs: http://tinyurl.com/omjkazg