Xubuntu 14.04: how to enable hibernate?

Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:

sudo nano /var/lib/polkit-1/localauthority/10-vendor.d/com.ubuntu.desktop.pkla

Look for

[Disable hibernate by default in upower]
[Disable hibernate by default in logind]

Change the value of "ResultActive=no" to "ResultActive=yes" in both, press ctrl+x to exit the file. It will ask to save the change or not. Press Y to save the change, if you're on Ubuntu restart your system.

Edit

Just tried this on Xubuntu 16.04, and Ubuntu 16.04. and it also works.

For Ubuntu 16.04 I had to reboot for it to work.