Stonehenge is right, you have to know that your cash is coming in, and that all are paying. You also have to know where your money is going. I charge out parts and supplies to each piece of equipment. I can then keep track of repair costs, and it figures into your cost per hour of operating/owning each piece of equipment.
I also agree with Stonehenge, unless I have to have a piece of equipment, I buy off season. Saves a lot of money.