Does your Apple Mail feel sluggish? Then this is the command you should execute from the Terminal (after making sure Mail is not running):

sqlite3 ~/Library/Mail/Envelope\ Index vacuum subjects;

This is a safe command to run; it clears out deleted emails and improves the overall speed of Mail.

If you’re curious about what’s going on when you do this, please have a look at the blog posting over at the guys from Hawk Wings, where the inner workings and effects of the command are discussed in detail. They also have a ton of other good information to make Mail more productive, so go check them out!