You can follow official documentation here.
Before enabling hibernation, please try to test whether it works correctly by running
pm-hibernate in a terminal. The system will try to hibernate. If you are able to start the system again then you are more or less safe to add an override.
To do so, start editing
Fill it with this
[Re-enable hibernate by default] Identity=unix-user:* Action=org.freedesktop.upower.hibernate ResultActive=yes
Save by pressing Ctrl-O and exit nano by pressing Ctrl-X
Restart and hibernation is back!
unity --reset to just reset the menu (note that this will wipe all your compiz customisations!).
[NE: O comando unity --reset foi removido no Ubuntu 12.10.]
Some users will then need to run sudo update-grub to get the hibernate option to be available in the power menu..