I am wondering how I should make backup in the best way in my situation. Please help if you enjoy it.

I have a backup station with a capacity of 20 GB uncompressed and I got 6 tapes.

I will backup a few directories in the local linux system and some of these are mounted samba-shares from 2 windows computers.

I have defined these sets in flexbackup.

homes: (hemkataloger i linux) ~10 GB
sys: (/usr m.m. - linuxsystemet exkl /home) ~5 GB
jonas: (några kataloger i /mnt) ~7 GB
joj: (ytterligare några kataloger i /mnt) ~1 GB

I am thinking about using flexbackup but if someone has a better solution I am open minded. I have used simple scripts before but I have felt a bit limited using them.

I have also been thinking about usin crontab for automating and make a full backup of one set while making incremental backups om the rest the same day (or night). I still don't know how to do this the "optimal" way. Any ideas?

You haven't given quite enough information for anyone to make useful suggestions. You need to analyze the criticality of the data you are backing up, and the importance of transparency in recovering the data. Because you are limited in your backup capacity, you are using a combination of alternating full and incremental backups. With only six tapes, it would be easily possible for the operator to misjuggle the tapes and make your backups unavailable when you need it the most.

I suggest you look at http://www.mikerubel.org/computers/rsync_snapshots/ for a recipe on how to use an inexpensive HDD to make multiple transparent snapshots of your backed up directories. You could then use your tapes to make alternating full archival backups for use in case of a disaster that destroyed both your data drive and your backup drive.