This is likely a repost since it's kinda known, but searching "proving grounds queue pause" didn't do it for me, so yeah.
Basically say you're in an LFR queue and need 5-10 minutes to go take a smoke, a dump, whatever. Just head to garrison, enter proving grounds, and that'll "pause" your queue (retaining your position). When you get back, simply leave the Proving Grounds and it will resume your queue with no fear of being dropped in the interim.