A cup of vinegar added to the rinse water may help prevent rashes by making the ammonia wash out more easily. Diaper services do this as a routine, and, in general, are excellent. Once a rash develops, try to change your baby more often and clean her more gently. Leave the diaper off a bit more during the day. The rash will generally go away by itself.
Diarrhea is a special case.
Diarrhea stools in some infants with diarrhea almost seem to burn the baby's skin. The most important thing to do is to change the baby immediately after bowel movements, and clean him with plenty of clear running water. If you can arrange it, allow the baby to go without diapers for at least part of the day to let the air reach the sore skin. Once the diarrhea gets better, the rash will usually go away within a few days. Sometimes babies get a yeast infection in the diaper area.