Every person is different, so while some people will be successful treating acne with OTC products, they won't do anything to help others. It just depends on the type and severity of the acne. For myself, and now DD15, nothing we tried worked until we saw a dermatologist. And, depending on your insurance, it can actually be cheaper to get prescriptions than OTC. We were just throwing our money away on products that didn't work, and DD was just getting more and more frustrated.
As far as diet goes, there's no one food that causes or increases acne. However, there is research showing that a high sugar, high fat diet increases sebum production, which can increase acne. So, while a healthy diet might help, but you don't need to give up chocolate, for example.