I'm unilaterally declaring November the 3rd International Backup Day, or IBD for short. Grab a blank DVD, memory stick or removable hard drive and take a copy of your important files, right now. This is doubly important if you don't know how to do a backup, or have never bothered.
JA Konrath recently posted about various backup and restore methods, so head over there if you're uncertain about the whole concept of storing a copy of your data.
If you're not sure whether it's time to backup or not, consider this: If your PC died tomorrow, what would you lose?
You might like to post about IBD on your blog if you believe backing up is important.
Windows users: At the very least you should take a copy of your 'My Documents' folder - right-click on it, select properties and note how many gb it takes up. You'll need a memory stick at least 10-20% bigger than the total, so 3gb of files will require a 4gb stick or a blank DVD. 500 megs of files will fit onto a CD.
If you have a lot of data, I recommend an external USB hard drive. Just remember not to leave it connected to the power or the computer when not in use, or a lightning strike will take it out along with all your other gear. If you DO pick up a 500gb drive, don't waste all that room. Make folders called Jan-Dec and schedule a monthly backup into the relevant folder.
If funds are tight you can pick up an empty 3.5" USB hard drive enclosure and fit an old 10 or 20gb drive. That's equivalent to 20-40 DVDs, which is masses of room for the average user. If you have a huge collection of digital camera pics, consider how much you've saved on film and developing, and put a tiny portion of that cash towards a backup solution.
Personally I'm a backup freak, with a dedicated 200gb partition containing automatic daily, monthly and yearly snaphots of My Documents, two dozen external hard drives, backups on two completely different computers and a pocket full of memory sticks. But you don't have to go that far.
Finally, a free backup app you might find useful. (Windows only)
Official International Backup Day page
Simon Haynes is the author of the Hal Spacejock and Hal Junior series (Amazon / Smashwords / other formats)