If your R134a is leaking out, you need to fix the leak. My car was in an accident a few years back and I had to have the A/C condensor replaced (big radiator looking thing) along with one of the lines between the compressor and condensor. I refilled the system with a kit from a local parts store and it worked for the summer. The next summer it was just blowing warm air. Apparently the body shop that fixed it (and replaced the orignal condensor) didn't check the clearances and the new rad support rubbed the top of the cendensor and cut a gash into it (I didn't notice the coolant started spilling out again). I replaced the condesnor with a different one and was going to trim the rad support but this condensor fits better so I didnt' have to and havne't had any trouble.

Long story short, if your low on refridgerant, you need to figure out where it went and fix it then fill it back up. The refill kits are only around $20-$30 (most come with a gauge, the tube with the fittings you need along with instructions and a can to refill) so you might try my way (fill and find the leak) or better yet, take it to a qualified A/C shop and let them figure it out.