Tape Backup Library Procedure

We are migrating from doing full nightly backups, on one or two tapes, to a 20-tape library. What is the best way to perform backups, & store offsite media? The goal is to be able to easily retrieve data from backups. To accomplish this goal, theprocess must be easy to understand.
We are thinking of doing full backups on Sunday, then incremental backups each night. At the end of the week, the tapes that have data written to them can be moved offsite. But out of a 20-tape library, how do we know which tapes have had data written to them?
How do other people use their tape backup libraries?