Most important is to keep the area dry and change the nappy several times in a day, especially in summer. Each time clean the area well with baby wipes, apply some baby talc to prevent friction. The idea is to minimize occurrence of the rashes.
For existing rashes any nappy rash cream, baby safe moisturizer or petroleum jelly works well.
I have tried many products from chemists and I finally found one product that works which is completely natural and certified organic ingredients. I have it listed as the first product in this list of natural baby products:
I know that natural is best, but when my son was teething a few years back, he had horrendous nappy rash and he was in constant pain. I tried all the natural remedies, tried all the creams, and nothing helped. We did nappy free time, changed his nappy frequently when he was wearing one, but the teething was causing such problems. Eventually a friend sent me a tube of something called 'Metanium' from the UK. It is a cream with titanium dioxide as its active ingredient and it cleared the nappy rash up within 2 days and my son never had nappy rash to the same degree again. It's not everyone's choice to use 'non natural' products, but after exhausting everything on the pharmacists shelves, this was a miracle product for me.