You use it after cleaning,i tend to use Carplan Demon shine works for me and i've used it as claying lube.
The more serious detailers will be along to give you more advice.
Autoglym have a new detailing spray out and after using it this week I can say that it is very good. Can be used for quick detailing or when claying. As a quick detailer, use between washes to remove any dust, not dirt or grime.