A nurse told me that in addition to lowering your body's natural defenses (which would kill off all the little yeasties), prednisone raises the glucose level in your blood and tissues (which the yeasties LOVE -- in fact, prednisone can cause problems for diabetics, and even CAUSE drug-induced diabetes). She said it's not unusual for women on prednisone to get constant yeast infections because of this.
Diflucan is usually the first step, but if that doesn't work, there are other options. Some yeast infections are Diflucan-resistant, and there are other systemic anti-fungals out there. It depends what kind are making you sick.