Pausing from the Operating System:

###Linux

if you are on Linux (not tested on mac os)
you can send a signal with:

killall -s STOP blender # to start it again: killall -s CONT blender 

Some advantages to this are that it will work for any task, not just rendering, and that you don't have to start blender from a terminal.

Windows

You can use process explorer for this - see this question on StackOverflow

Pausing from Blender:

Pausing F12 renders from Blender is not implemented yet, however it is on the Cycles Todo list.