SQL Server's transaction log is the key to being able to recover from a variety of system and software problems. As a general rule, a database should have its transaction log backed up at least every day. Any database that is undergoing a heavy transaction load may require more frequent backup. The rule of thumb balances acceptable risk vs. performance hit. If you back up several times a day, then try and do your backups at times of low transaction activity.
Another technique used by administrators is to mirror the database and do backups from the split mirror. When the mirror is reestablished, changes from the primary mirror will be replicated over time.
Remember that you don't need to back up all of your database transaction logs. The master, model, msdb and distribution databases logs don't need this kind of backup routine, although they do need to be backed up from time to time (particularly the master database). You also can omit any database transaction log where the option truncate log on checkpoint - true.
Barrie Sosinsky (firstname.lastname@example.org)is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.