1. Wear clean shorts every day.
2. Wash the “rash prone area” with soap and water every day.
3. Get out of your soggy shorts as soon as possible after riding. (A kilt would work great for drying parts out, but I’ve haven’t seen anyone willing to wear one.)
4. Use a lubricant/protectant. No, not for your bike, for your fanny. You can buy cycling specific products at the bicycle shop. Examples are Chamois Butt’r, Body Glide, ASSOS Chamois Saddle Cream, and Sixtufit Buttock Crème.
However, I find it easier and cheaper to use products from the drug or grocery store. So, go to the infant care department and look for some diaper cream, since your problem is probably due to sitting around in sweaty shorts all day. There are dozens of products, allowing every cyclist you know to prefer a different product. Examples include Balmex Diaper Rash Ointment (my favorite), Desitin Diaper Rash Ointment, petroleum jelly, A+D Ointment, Burt's Bees Baby Diaper Ointment, Bag Balm, Johnson's Baby Diaper Rash Cream with Zinc Oxide and similar store brand products.
Find a product that sticks with you, but also feel free to add additional layers of crème during a long day in the saddle. And, you want to be able to wash the stuff out of your shorts, too.
You can use one product by day, and another at night (petroleum jelly works well at healing over night). Also consider an antibiotic cream such as Neosporin, Bacitracin or a store brand Triple Antibiotic Ointment. Hydrocortisone Anti-Itch Cream can be useful, too.