I've always cleaned my new and used kegs the following way which used a minimal amount of CO2 to push a little bit of fluid through the diptube, beer line, and and tap;
-Hot soak w. PBW (followed by a quick flush through the lines and tap)
-Rinse out the keg with clean water (another quick flush of this water through the lines)
-Sanitizer in the keg (another flush)
I've seen some of the cleaning kits online that include a hand pump spray bottle. Does anyone use these? I'm wondering if it's more trouble than it's worth since I'm really only using a small amount of gas.