I've been using
slock1, from the suckless tools collection, to lock my computer recently. And I gotta say, it's really doesn't suck. Using it literally couldn't be any simpler: at any command prompt (ie, in a terminal emulator, awesomeWM's "run" box), you type "slock".
And that's it. Your computer is now locked. Mouse input is ignored, to unlock it you simply type your password at the blank screen. There's no password prompt, no screen saver, no options to mess with, nothing. Just your locked computer and a blank screen. If you're worried about burn-in, turn your monitor off---you should be doing that anyways.