Diaper Rash

My son is 4 months and has really sensitive skin. I always change him when I notice he has a wet or dirty diaper. But during the night he will go longer of course and I always put Desitin on him when he goes to bed. He still sometimes wake up with a small rash and I just let him go nakey for a good part of the day. But today He woke up with a pretty bad diaper rash. Any suggestions on what to do to keep him from getting these rashes?