I have the exact same problem! Like someone else said, too, by the time I get to the end of a bag it seems like it takes a lot more to get me high. I've gone from smoking a quarter a week to a half an oz in less than a week, which just isn't sustainable for me due to the prices around here.
It also seems like I can only really get high the first time I smoke in a day, so I've been trying to wait until later in the day to smoke - but man, it's hard!
I just finished up a 1/2 oz I bought last Friday, and there won't be any more until at least Saturday. What I'm going to do is ration it when I get some more. Like one joint per day or something. That way it will last me longer, and maybe my tolerance will come down a bit.
Quitting for a while is really the only way to lower your tolerance significantly. Cutting back, waaaay back, might be good enough though if you don't think you can quit for a while.