Go to...
DrsFosterSmith.com
or call them at 1-800-826-7206. They have a few products you can use on existing brown spots. They also have a couple different "treats" you can give your dog that acts as a neutralizer against the ammonia in the dogs urine. I've given these to my lab/rott and it worked real well. The product I bought, and would recommend, is "Lawn Guard Treats". Give them (quantity of treats based on dog's size) as a treat in the morning. I didn't notice any change until after a few days of giving him the treats.
Similar product may be available elsewhere...?