You go into the $FWDIR/log folder/directory and FTP them on another server for storage there are 4 files associated to each log so get them all. Then you can delete them, and if you need them back just FTP them back into the same folder. Occasioanlly, I've had the file get corrupted and the I can't open them after bringing them back onto the FW.
The initial FTP process can be scripted to run automatically, which is what we're done.
good luck