First and foremost, you must have sufficient swap space for hibernation to work. The exact amount varies depending upon your system, but a general rule is for your swap to be 1.5x to 2x the capacity of your RAM.
Assuming you have sufficient swap space for hibernation, edit
add the following:
[Re-enable hibernate by default in upower] Identity=unix-user:* Action=org.freedesktop.upower.hibernate ResultActive=yes [Re-enable hibernate by default in logind] Identity=unix-user:* Action=org.freedesktop.login1.hibernate;org.freedesktop.login1.handle-hibernate-key;org.freedesktop.login1;org.freedesktop.login1.hibernate-multiple-sessions;org.freedesktop.login1.hibernate-ignore-inhibit ResultActive=yes
Reboot and you should now have hibernation as an option.
Thanks to Aditya Tiwari on FossBytes for the original instructions.