Getting control of email so it doesn't control you, is easier said than done. One of the problems I run into is having the email client open on my desktop all the time. When I'm coding, I frequently just close it all together.
Sometimes though I want it open and want to delete things that can be deleted and reply to things that need to be to be replied to and ignore the rest. Mini Mail is an application helper for Mail.app that shows a tiny window with just enough information.
The best way to describe it is to compare it to iTunes. When you click the green window decoration, iTunes shows a full window, or a tiny control panel. Mini Mail does the same thing.
At first I was skeptical that I'd like it or use it, but now I love it. You can try for free for 30 days, so there's little risk.