Depends alot on your area and your yard (and you). There is a demand but also lots of competition. If you have space the best thing you can do is stick an ad in your local tack shop or somewhere similar (they don't usually charge) and see what happens.
I think there is definately a demand, especailly if you are offering schooling or part livery, in south wales area anyway a couple of friends do it, they got their names about by doing things like clipping and short term schooling and selling problem horses and a bit of teaching. You will be amazed how quickly people hear of you that way pony clubs and local hunts also tend to have people looking for these sort of services.