Well I would start with water changes. I don't know how extream your ammonia problem is (how high is high?). You may want to check the water you are putting in the tank as well. I was having trouble with nitrites for a while till I found out that my tap water had high nitrites, so all the water changes in the world would not have done me any good.

other then that what are your fishes symptoms? Sometimes a high level of some thing can stress them and make them suseptable to other illnesses.