The easiest way to dump it is to go all the way down, or close to down, then throw your head back and drop your shoulders and pull them forward. The bar will fall down your back and you'll spring forward naturally and get out of the way. the real key is throwing your head back while pulling your shoulders down/forward. Get the bar to slide down your back and it'll push you out of the way.
Also, obviously you can't dump the weight the way I'm talking about if you are falling forward. I've never dumped a squat like that, so I have no idea what I'd do.