* Amount of "static" data and data frequently accessed and modified
* Data security and handling policies
How valuable the data is
Data loss is not just concerned with loss of files. It's a loss of efforts, money and time. In business it's a loss of trust of both employees who depend on stability of the system and partners who can't wait for you to recover your data. At home it's a loss of nerves ? just think of your memorable videos, or of all your favorite applications that you use daily and have tweaked to perfection. When thinking of a backup routine, don't underestimate what you have. But also don't overestimate it ? keeping a complete hard drive backup when you could use a single CD is not an effective use of your resources.
Amount of "static" data and data frequently accessed and modified
It's wise to divide your data into several blocks that you will process separately. Say that you want to back up your photo archive. Browsing thousands of files and determining which of them are new and which were modified is not something that should be done hourly. At the same time it's a good practice to make backups of databases used in commercial environments as often as you can afford ? in order to avoid any mistake and be able to instantly roll back to any desired state in the past.
Data security and handling policies
Another parameter by which you should differentiate your data is its level of privacy. It's reasonable to protect sensitive and personal data from third-party access with passwords or encryption. Most important commercial data ought to be kept in a different place than all "ordinary" backups, secured from any unauthorized access.
After you decide which blocks of data will be processed in which way, it's necessary to think of storage where they will be kept. See the next article: "How to choose backup storage media"?
