I grew up washing and waxing airplanes; it's how I worked off and paid for my initial flight training, in fact. I did a lot of it in an unheated hangar in the winter. Often the water would freeze on the airplane as I was washing it, and would come off as ice on a rag when I wiped it back down.
I've found that keeping an airplane clean by washing it after each flight is the most effective. It can be done without a full bath, or even a bucket of water. A squirter bottle with a wash n' wax type soap, sprayed on and then wiped off works really well. Even on airplanes that fly into really dirty environments, it works well...and by reapplying the wax coat every time, it makes cleaning again a real snap.
I used to fly a large four engine bomber that got cleaned after every flight. A full cleaning took about fifteen hours. As you can imagine, anything that could be done to reduce that time was a real plus, because I didn't get paid to clean...just to fly. It was a radial-engine powered airplane, which meant a lot of oil and a lot of mess. Keeping things as clean as possible, keeping things waxed, and chasing leaks and dirt before they got out of hand kept the job simple.
I had to carry my cleaning supplies with me. When able, I cleaned the airplane with a firehose, but the airplane was in the field a lot, and flying through ash and smoke and all sorts of things that made it dirty. Keeping a coat of wax on it was a big help, and a squirter bottle or garden sprayer with an automotive type wash and wax solution, usually in concentration, worked really well.