I don't run a boarding stable so can't help too much, but I've learned a couple of things from watching how my barn is run. One is that it's very hard to make a living doing it. The second is to not make "exceptions" for anyone. Don't be penny grabbing at every opportunity, but make sure you charge for what needs to be charged for, and make sure you get your money. My barn manager makes next to nothing because she is keeping about four of her own horses in stalls, two of her sister's, and several other boarders have some sort of discount- not because they work off board, but because she wanted them to be able to keep the horses when they otherwise wouldn't have been able to afford board. Kind of her, but not good for business.
Operating off the assumption that the facilities are in great repair and fully operational, there are still lots of things to consider. The big one is deciding what services you want to include. Will you be providing full-service board with stall cleaning daily, grain and hay provided, blanketing, worming, calling vet and farrier, etc? Will you just have the bare minimum "full service"- stall cleaning, feeding of owner-supplied grain/hay- and charge for extras such as blanketing? Will you offer self-care options, or pasture boarding? Is there an arena or some sort of suitable riding area, or will the property owner work with you to build something? Will you bring in a trainer? Will you allow outside trainers? If so, will you charge a fee?
Just a few things to consider. Like others have said, make sure you have a very good boarding contract!