I have never done it myself, but I know that Wal-Mart sells a "do it yourself" recharge kit for R-134a systems. I belive they cost between $15 and $20 - and one type has a gauge on it that tells you how much to put in. I spoke with a fellow customer who said that it worked well for him and lasted the rest of the summer.....versus having to fork out $200 to the stealership.
Apparently, you hook it to the low pressure side of the A/C unit where there is a little valve, while running your system at MAX power. Just press the button on the can - much like a flat tire repair kit; and wait until the needle is in the green zone.
Good luck to you and let us know how it turns out!
P.S. Just don't use it on a car equipped with R-12 (pre-1994 vehicles, mostly)!