Any skinfold yeast infection can be very tough to defeat, especially if you live in a hot moist climate. Keeping your skin cool and smooth and dry, even with powder, can certainly help prevent problems, and may be all you need to clear up a simple heat rash; but if yeast has moved in for a while, you'll probably also need drugs to shake it.
If the infection isn't clearing up and you're sure it involves yeast, you might first try an over-the-counter topical antifungal cream containing something like clotrimazole or miconazole. Check the foot powder section in a drugstore or supermarket. If the first one you pick doesn't work after the recommended period, you might try one with a different active ingredient; some yeasts respond better to particular antifungals.
If a couple of different antifungals don't do it for you, it's definitely time to see a doctor.