Just choose something that's not in a dictionary, isn't too short, and preferably has a couple of numbers in it. The only way to break fSekrit containers would be bruteforcing them... I think there was a table posted here somewhere with some rough estimates of how long it would take to bruteforce varying keyspaces. Personally I go for ~10 chars or more, depending on how secure the stuff needs to be.
Btw, ever since v1.2, saving identical data with identical passphrase does not generate the same output files (randomized IVs).