Q:

Xubuntu 14.04: how to enable hibernate? 

I'm using Xubuntu 14.04 64 bit.  I know that question been asked a billion times, but all of them consist of editing following file:
/etc/polkit-1/localauthority/50-local.d/com.ubuntu.enable-hibernate.pkla

That file does not exist on my system.  I tried com.xubuntu.enable-hibernate.pkla but doesn't exist either, locate command doesn't find it. 

A:

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.