Use some good clumping cat litter like Scoop Away and scoop the litter box regularly. I had the same problem you did with this crappy Tidy Cat litter I bought for a while, because the clumps just fell apart and I couldn't scoop them out, resulting in some serious ammonia odor. I switched to Scoop Away and haven't had a problem since.
I usually don't need to totally change the litter, but I scoop it out every couple of days and have no problems with the smell, even with 3 cats using the litter box regularly. I use Scoop Away Multi-Cat Formula, by the way and it has a nice scent to it which probably helps with the smell. A big bucket of the stuff usually lasts about a month and a half before the litter level starts getting low in the box, then I just refill it. Just make sure you do a good job of scooping everything out each time by digging through all of the litter down to the bottom to ensure you get everything.