In the business world there is a saying that you should always under-promise and over-deliver. That means make sure your clients know what to expect, and if you surprise them, do so in a positive manner.
Make sure you have boarding contracts and that they spell out exactly what is expected of you and the client. Same goes for lessons or breeding arrangements. This is not only a great legal way to protect you and your interests, it ensures that you have the opportunity to discuss anything that might become a problem.
For example, if your client thinks that you will be available to bring up her horse and hold it for the farrier and vet as part of the boarding agreement, and you charge extra for those services, having that out in the open when the boarder is joining your stable will ensure there isn't a dispute later when she sees the added service charge on her bill.
If you close your arena at 10 p.m. in the summer and 8 p.m. in the winter, then make sure that is spelled out, too. For working horse owners that might make a difference in choosing your facility.
Those boarders who constantly ask you for "favors" can be controlled if the responsibilities are spelled out in your boarding contract. Asking you to put on or take off blankets, "I can't make it out so could you clean Johnny Boy's stall and feed him" episodes, and "Can I borrow the truck and trailer" requests can all be addressed and managed.
If you occasionally, out of the goodness of your heart, want to do some of these things and not charge for them, that is certainly up to you. But beware of the clients who assume if you do it once for free that you'll do it again (and again, and again) for free.
Manage your clients' expectations and you will find that not only are you and the client much happier, but that you are earning money for the jobs that you do for the client.