Spray bottle of water has always worked for me. Just do a thorough job of making the nearby wood (joists and subflooring) soaking wet. Then give it some more water if it steams itself dry while you're soldering. I've noticed over the years (and even recently on a few DIY TV shows) that people have a tendency to use a propane flame that's way too large for the task--just a pencil-sized blue flame, 3/4" long and slowly moving, will get the job done. And never direct the flame at the junction points where the solder will enter the fitting, but rather at the mass of the fitting itself.
Brings back memories of flame-straightening (acetylene) damaged bridge girders. Just a small No. 2 tip, properly applied by someone skilled in the practice, is enough to bring a severely bent and warped girder (looking like it's ready for the scrap pile) back into line.