I second jaredw's advice. I've used Western Digital 1 Terabyte external drives as backup for several years now. After getting a drive, the next thing you need to do, is decide on which backup software to make your backups with. These external drives come with backup software usually. Try that software first to see if it works for you.
After you make your first backup of everything on your laptop, you'll want to run a differential backup of changes ideally once a week at least. If the time it takes to make the differential backup is inconveniently long, say more than 30 minutes or an hour, investigate buying backup software. Real backup software (such as Retrospect v. 6 for Mac) is incredibly fast and reliable. Don't use any of the free or shareware backup programs - they are not reliable when you need them to be.