How to get the most out of your ad agency

They say that bad clients get the work they deserve. I spent the last 7 years working for creative ad agencies working on briefs of all sizes from a wide variety of clients, and let me tell you I've seen some stinkers. Sometimes we worked with makers who had great products but no clear idea of what they wanted their ad campaigns to say or their audience to think. Other times we worked with clients who had no idea how to judge the creative they were paying us to come up with, which led to frustration on both sides. If you work with an ad/marketing agency I'd love to hear your experiences (good and bad). I'm happy to help makers who want to get the most out of their client/agency relationship or want advice about taking the plunge with a new agency.