Do it regularly. Set aside a time to back-up every week. Be realistic about your needs. You can fit a lot of data on a lowly floppy disk. If you only have a couple of files to backup, just copy them manually to a floppy from File Manager.
Zip drives are often a waste.
Despite the hype, if all you need is backup, cheap tape drives have recently appeared on the market that give you a better per MB price. Call your Systems Office rep before you lay out money for removable media. If you are responsible for an entire department's centralized backup, you definitely should talk to us. Think creatively about automation. You can write simple batch files to copy selected files to your backup.