Before you do anything, you will want to check the pressures in the a/c system. IT is usually best to check the low side, and the high side pressures, at the same time, as this will tell you if the a/c system is working properly or not, or if just low on refrigerant.
What you described, mine does exactly the same, this is how I now I need to squirt a little refrigerant in mine. I get a can (32oz) at the auto parts store that has the little gauge and trigger on it. When I put it on and check it, its usually is about 20psi on the low side (valve on fire wall) when the driver side only blows "cool" and the pass. is side is ice cold. I put enough in to bring up to about 23 psi and then it is comfortable again. The system has 192,000 miles on it!
If you a/c system pressures are normal, then you will have to start checking something else like the blend doors and such.
