Actually baby shampoo is not good for guinea pigs - it dries out their skin. A small animal shampoo (many use Four Paws Bunny Shampoo) is much better for them. Some people use shampoo formulated for kittens, but you have to make sure there are no pyrethins in it.
Some guinea pigs never get a bath - too many baths can dry out their skin. check out this link
Guinea Lynx :: Grooming
I'm not sure what spray bottle stuff Sammy is talking about, but I would want to make certain it was safe before using it.