Prune Juice has never worked for me, but I think you can/should drink as much as you want. Just drink a cup or two and wait for an hour and see how you feel. Maybe you should also try mixing a spoonful of Benefiber in with it. :) It's a natural, tasteless fiber supplement and a long term laxative. I drink a cup of it nightly and haven't experienced constipation in quite a while.
If all else fails, drink a big cup of black coffee, smoke a cigarette, and go for a jog. That'll get the pipes working.