What is the best way to cure a bad diaper rash?

Keep 'em clean: Prevention is better than cure. Remember to use small amounts of baby powder, since larger amounts tend to hold the moisture next to the skin instead of allowing the diapers to do their job. For a cure, meticulous hygiene, allowing the area to air dry, and using light amounts of over-the-counter products usually works. There are also prescription cures which you can get by seeing your doctor.

Topical agents: There are a host of topical creams or ointments which will help (ask your pharmacist), and keeping the area as dry as possible will also shorten the duration. If it turns out to be a yeast however, it may require different medicaitons, and if it does not clear rapidly have them seen by a family doctor or pediatrician.