I bring this topic up because I read a post on Phil Windly's blog talking about receiving a lot of email (not spam) and trying to respond to it all. I do agree that no matter what you do, you probably will never keep up with all your email. However, as a teacher, I can really use Tantek's advice. Whenever you receive an email question, you could post it on the course message board, along with your response. Then when you get that question again, just refer the student to the board.
In theory, students will look at the board and see that their other questions were answered on the board, and then start checking the board before emailing their professor. Hopefully, the class could transform a previous unused and unnoticed message board into their first source for help.
To make the process easier, it would be really nice to have a drag-and-drop interface to just drag your email conversation to the message board. Then, for students with similar questions, you could drag their email to the post, which would automatically reply to their message with a link to that post. Maybe that will be my next project.