The basic method for controlling odour in cat litter boxes is to strain frequently for solid matter and change the litter often so the ammonia from the decaying urine does not become a problem. If the urine is left it will go into a second stage of decay and a mercaptan odour will be produced which is very unplesant.
Odorex® Animal Odour Eliminator (A.O.E) which is often used in veterinarian hospitals can be sprayed on pet litter with very good results. It will control the odour and extend the life of the litter. Using a good quality pet litter like PAWS which is super absorbant and controls odours will also help.