The most important decision regarding backups is the choice of backup medium. You need to consider cost, reliability, speed, availability, and usability. Cost is important, since you should preferably have several times more backup storage than what you need for the data. A cheap medium is usually a must.
Reliability is extremely important, since a broken backup can make a grown man cry.
A backup medium must be able to hold data without corruption for years. The way you use the medium affects it reliability as a backup medium. A hard disk is typically very reliable, but as a backup medium it is not very reliable, if it is in the same computer as the disk you are backing up. Speed is usually not very important, if backups can be done without interaction. It doesn't matter if a backup takes two hours, as long as it needs no supervision.