Sometimes you just don't have a pair of dice and a shoe box to generate your passwords. To cover that need, diceware was born. diceware is a small utility to generate diceware style passwords.
Just download diceware and make sure you have the following utilities installed:
hexdumpgrep
diceware uses /dev/urandom to gather the random bytes it needs to pick the words from the word list. All the above should be available on any (modern) UNIX machine.
A word list can be downloaded from the EFF. diceware only supports word lists with 7776 words on it (5 dice rolls).