Alright, I've been here, done this twice. Your compressor is probably just fine if it hasn't been making any rattling or squealing noises. It is possible to develop a leak in the system somewhere, but the O-ring seals dry up and start to leak over time also.

The system only takes about 1.5 pounds of R134a, which isn't much at all. The amount of refrigerant is crucial, as is the amount of oil in the system (PAG oil, do NOT put ester oil in!)

I recommend that you do this:
1) Get a set of o-rings and a new receiver / dryer, some brake cleaner and HFCF-141B flushing agent. You will also need a gauge set and a good vacuum pump, which you might be able to rent.
2) Remove all the hoses, one at a time, and note where they go.
3) Remove the condensor and dryer. Pitch the dryer in the trash.
4) Flush the lines and condensor with a good A/C flushing agent. (I use brake cleaner followed by HFCF-141B, then blow it out with compressed air.)
5) Install the new receiver / dryer with new o-rings. But before you do, make sure you fill with the proper amount of PAG oil, as listed in the manual. Make sure that the new dryer had the shipping plugs installed, or else it is ruined and shouldn't be installed.
6) As you install the lines and condensor, (with new o-rings), they get PAG oil too.
7) Check belt tension on the compressor
8) Hook up a gauge set on the high side and low side ports, and use a vacuum pump to draw a vacuum. Check to see that the system is holding a vacuum. If it is, then keep drawing a deep vacuum for about an hour to boil out any water in the system.
9) Use a R-134A refrigerant that has nothing else in it, except for a UV dye. (no seal conditioner, leak stop, ester oil, none of that crap)
10) The initial fill is crucial. With my fill, it was 2 ounces shy of two full cans. 2 ounces is just about enough to slosh around in the nearly empty can. If you go off the gauges, the high pressure gauge may take a bit to settle out and you find that you've added too much. No problem.
11) If you find you've added too much, then do this: Shut it off, close the valves and let the pressure equalize. Put the can in an ice bath for a bit to cool it off. Once the high pressure side has dropped to a safe level, open the valve back to the can and squirt a bit of refrigerant back into the can. (That is why the ice bath, cool the can and the pressure in it drops)

Finally, if after all of this the system should work, as long as the compressor is functioning and the fan clutch engages (which you said it does). If it works for a while, then starts to blow warm air again, then great, you have a leak. This is why you put in the UV dye type refrigerant. Get yourself a black light and start looking for the source. I haven't heard of any leaks in the evaporator on these, but it could happen. For that, look for the dye coming out where the water drain is.