A good tool to generate strong passwords and secrets is "apg". You can get it with
sudo apt-get install apg
To create a strong secret for sessions, hashed Paperclip paths, etc. say
apg -m128 -a1 -E\'\"
Arguments explained:
-m
parameter defines the secret length-a1
makes apg choose from all 7-bit ASCII characters instead of just the alphabet-E\'\"
excludes quote characters so you can easily paste the secret into a Ruby string