Dawn dish soap for bathing (the blue kind). Or, if your Wal*Mart has a pet section they should have whitening shampoo for dogs.
For pre-show ring prep, I prefer to use chalk or shapley's spray, but since you're in a pinch just pat a bit of baby power over any white.
For things like socks and blazes, clipping always helps get the really stained hair off. Wrapping legs the night before (if you bathe the night before) and not taking the wraps off until absolutely necessary always help too, of course.