Like others have said, cutting down on sugar is good because thrush - that coating on your tongue - is caused by yeast, which loves sugar.
The reason people get thrush when on antibiotics like Flagyl is because the antibiotics hit not only the bad bacteria, but also the good bacteria. Flagyl causes the ratio of bacteria to yeast to go down which can cause an imbalance, since both yeast and bacteria must coexist in balance in your intestines.
So...this means that you might benefit from homemade yogurt, which is far more powerful than store bought. Also, to replenish good bacteria you can take any number of probiotics. For instance, Solgar's advanced 40+ acidophilus.
Cutting down on sugar and probiotics will help Nystatin do its work. But don't take Nystatin for more than a couple of weeks, because, as I recall, Nystatin interferes with the absorption of many nutrients. Diflucan is a very strong, very effective antifungal med but Nystatin is far safer.