its best to back up once a week, that way after you delete your messages and realize you need one, you can go restore a backup so u can grab the message you need.
Or if you have your email messages still in your email client on your pc you could forward them back to yourself. Just adjust your receive time on your pc email client long enough for your BB to get the messages or shut down your pc email application after you send the forwarding messages until you get them on your BB.
Posted from my CrackBerry at wapforums.crackberry.com