Twistory, is a new Twitter-app site that lets you back-up your valuable tweets on Google Calendar through Google API. It’s simple to use and all you have to do is enter your Twitter username to get started. Of course, if you use your Google Calendar for other purposes, we suggest you to make a new Twitter-backup account for Google Calendar so it stays separate.
I find this the best way to back-up your tweets since Twitter itself doesn’t offer any kind of backups.
Best of all, it’s free so you can lose!