I'm big on trying to keep an empty inbox (19 in there right now). I'm not so much a fan of folders to process incoming mail because they tend to squirrel away things as "filed" that still need to be done. But as Laura says, there's nothing worse than a piled-up inbox. So here's my strategy:

1. Turn off auto send/receive option.

Your inbox is not the boss of you. Check your email when you're ready. If it's an emergency, there's this weird thingy called a phone. Otherwise, it can wait 2/3/4 hours until you have time set aside in your day to process emails.

2. Have a quickie.

Yes, a poor attempt to make quick responses sound sexy. But it is rather fun to whip off all the 30s. replies and hit the 'ole delete button. Or, nearly as satisfying, file it away in whatever folder system you have.

3. Everything else gets turned into tasks.

This is why I will not exist without a PIM that incorporates email. When a response is going to take several minutes, or needs information from somewhere else, or otherwise would necessitate that email lounging around my inbox for more than a day, I make it a task. If I'm in Entourage, I use a "Create Task from Message" script (assigned to the shortcut key ^T) (darn, I've forgotten if it was built-in to E or if I downloaded it from somewhere). It pulls the subject line and body of the message into the description and memo of the newly-created task, which I then contextualize in the spirit of GTD and assign to the appropriate category (i.e. ".Waiting For", ".Errands"), and date it. Now it's out of mind, ready to be "done" when the context hits me on the right day.

Hope that helps!