If you hunt waterfowl with some regularity, you probably have at least a dozen decoys stored in your barn, or your garage, or under the porch at your duck camp. If you are really over the top (like me) you may have several hundred decoys at your disposal. Some duck camps will have several thousand on hand. In this day and age, decoys usually means plastic decoys, and if you hunt long enough with them, or leave them out for the season, you are going to find that eventually they will get dirty, and when they get dirty they are far less effective.
This is the time of year I clean decoys, and a rainy day is a good day to do it. I used to scrub, and spray, and work to get decoys clean. One year I left them all in the bed of my truck and drove the whole pile of them through the automated car wash. While it was a mediocre effort for the duck decoys, the truck looked good afterwards.
After cleaning filthy decoys for 30 years, I think I have settled on an easy, effective, solution. Get a jug of 30 Seconds Cleaner at your hardware store, and put it in a spray bottle. Wearing rubber gloves and glasses, spray the cleaner on the decoys and let them sit. Given time, the cleaner will eat away the mud, yellowish water stains, and discoloration from the decoy. Rinse with hose, or let the rain rinse for you.
Following that procedure, and once dry, spray the decoy with aerosol tire cleaner. It will cover your decoy in white foam, and when the foam dissipates, the decoy will shine like new. Actually–it will shine too much, and you will at first fear that it is too shiny, but having done this many times, I can tell you that the shine dulls off quickly, leaving a duck or goose decoy that you will be happy to hunt over.