The best cure for diaper rash is prevention. Make sure that your baby is getting enough water. For nursed babies that means that the mother must be drinking lots of water and juices, especially in hot weather or when she is ill. Your baby should have at least 4 wet diapers a day, and their stool should not be hard and dry. Babies that are drinking from cups or bottles should always have liquids available.
Water is the best for the other end as well.
Rinse with water often. Soaps are usually not necessary as the stuff in a diaper is all water soluble, and soap can dry the skin. When you do use soap, rinse well. Keep a little wash cloth wet with water in a baggie in your diaper bag. Just like the rest of your baby, the bottom will be healthier for a good dose of sun light and fresh air.