Back up all of your email using a POP3 or IMAP protocol (how?). Read more on this user who allegedly had his entire Gmail account deleted. The story made my stomach hurt a little. The comments are also pretty interesting to read.
If you’re new to POP3 and don’t currently have a program that can be used to read, send, or store email locally on your hard drive (or external hard drive) I highly recommend Thunderbird. It’s free, and because it’s made by the same people who make Firefox, it’s very reliable, stable, and feel goody.