Be specific about how much time you want from the senior vs. the juniors. Don't isolate your company's best people from the consultant. Don't send the consultant off to junior personnel just because your more senior folks are busy running the business. Don't accept arrogance from your consultant. A good consultant is not out to score points at your expense.
Smart consultants understand that the client's success is their success.
Do suggest that your consultant consider a contingency fee arrangement where the consultant's fee is paid based on the amount of money saved. Most consultants won't like it because they fear that the company will screw up their recommendations. Most successful small businesses will eventually need the services of an outside consultant. It makes sense to hire one if there is a legitimate need that cannot be addressed internally. But make sure the need is defined and that the objectives of the assignment and the relationship are clearly understood before you pick up the phone.