First, be certain that it is a yeast infection and not a bacterial infection. This misdiagnosis happens a lot in adult women, so I imagine it could happen in little girls as well.
Avoid soaping inside the vaginal area. Just rinse with clean water...let nature do what it is meant to do.
No bubble baths. In fact, skip the bath altogether and just rinse her while sitting on a little stool.
Completely dry the area before applying any treatment. I used a hair dryer throughout our diaper time and managed to avoid using too much diaper cream and healed what we did have very quickly.
If she eats yogurt, make sure it is good active culture stuff...if she isn't eating it, consider adding it. Cut down on anything sugary...including juice and fruit.
I also have broken open an acidophilus caplet right on the the rash before. Can't swear it made a difference, but did it on some advice.