Rule #1 Boarders are not your friends. This is a business. Run it as such.
Now if you don't mind getting run over, make friends. But just know if those friends find a cheaper more convenient place, you won't see much if anything of them afterwards.
Rule #2 do a cost analysis then add 20% to cover drought and blight on hay crops. Hay ALWAYS goes up when it's dry. Grain prices too. Anything you don't spend, rebank or buy extra hay when the prices are right. Consider buying bulk amounts when you can get it on a deal.
Rule#3 Fences can make or break your business. If you don't have awesome fencing, and maintain it then your clients will find elsewhere to go. No one wants to pay vet bills because of broken metal gates, downed wire, and the other 2003 things that can go wrong because of crappy fencing.
Rule #4 Insurance, insurance, and then some more insurance. Natural disaster insurance, fire, flood, liability, workers comp. it all adds up, but a lot of the premiums can be deducted from your taxes.
Rule #5 Power bills can kick your profits in the ass rather quickly. If you are going to light up everything (gotta be done) let your boarders know if there is a marked increase in rates, then it will be reflected on the board bill. This will help you by boarders turning off tack room, wash rack, and arena lights. Post signs about turning off lights when not in use. Consider shatterproof sky lights in the barns and arena for taking advantage of natural sun light. You could also consider using solar power since barn roofs are the perfect place for those huge panels to hide ;)
Rule #6 Cost of living increases. It happens. You set prices and 3 years down the road you are wondering where did the original profit go??? Check costs, then notify boarders of increases. Or make those terms clear in the original board agreements. Price your board accordingly.
Rule #7 Expect some turn over. Especially after price increases. It happens, and you need to bank against those times.
These are just some I thought of off hand, holler if you want more ideas LOL
"Light travels faster than sound. That's why some people appear bright until you hear them speak."
Larry the Cable Guy