If you're handy and have a little time, BigKatuna and MattColie have the plan I follow. Clean, oil, test... Clean the Coils and Blower, Oil the Motor, Test or Swap the Capacitors.
I'm blessed to have a DVOM that tests capacitors. With that and my cheap Harbor Fright AC Only Ammeter (plus one of their little free DVOM's) I can do a goodly amount of troubleshooting. I replaced the blower motor in ours (Coleman Mach 15) for about $100. It's never needed capacitors, but I keep spares for those on board. Anything "Sealed", the coils, compressor, even lines, I'd replace the unit.
