MySQL Backups - Full vs incremental - innobackupex
No, you don't need to prep every backup. You only need to create the full backup on Sunday and then create the incrementals from Monday to Saturday.

Full:

innobackupex --user=USER --password=PASSWORD /path/to/backup/dir/

Incr:

innobackupex --incremental /path/to/inc/mon --incremental-basedir=$FULLBACKUP --user=USER --password=PASSWORD

innobackupex --incremental /path/to/inc/tue --incremental-basedir=/path/to/inc/mon --user=USER --password=PASSWORD

The prepare phase (--apply-log) is only needed when you restore the backup.