Sounds like you're doing great already! I used ride a white horse and the brown dirt/poop stains on his legs were the worst. Go to a tack shop or search on line for some horse coat whitener or stain remover for horses. I used a solution of quiksilver and water in a spray bottle. It's a shampoo, but you are using so little in the bottle that you don't need to rinse it out with any more water. Just spray the stuff on and rub it around with a towel. =)
Also when you're grooming, don't forget the mane and tail! Brush out dirt and pick out pieces of hay or leaves or anything.
And you know what they say: If you're good at something, never do it for free!
Although I doubt many people will pay for grooming just on a regular basis, but you could master other skills like braiding manes and tails or special pre-show bath and grooming and people who show frequently may find that you do a better or faster job than them, and might pay you for it. XD