If it's been painted, it can be really messy to clean off. If you go to a good lumber yard, you should be able to find 1/4" drywall...I'd slap some of that up rather than trying to clean all the old crud off and fix the divots you make in the process.
For areas that haven't been painted, A spray bottle filled with plain water works. Spray it on, let it sit for a moment, then scrape it off. Works pretty well...I did about 500 sq ft in a few hours.