External Hard Drive: By far the easiest way to back up your data is to grab a big fat 1-2TB drive and moving all of your stuff onto there. If you've got a Mac, use Time Machine to get some version control of your files. It's best to set the back up schedule for at least once a month.
The Cloud Service: Also a ridiculously easy way to get your stuff up - simply install the software, pay the $5 monthly fee, and sleep safely knowing that your data's going to servers that are protected by militarized forces... okay, maybe not that last part, but it's good a good idea to have your data stuffed somewhere far away from your laptop. You know, just in case the would-be thief decides to take your backup drive as well along with it. MORE BACKUP OPTIONS: What's the Best Method to Back Up My Stuff? How to Securely Back Up Your Data Computer Maintenance: Backup Every Few Months Survey: How Do You Backup Your Files? [Via Cnet]