Money is power. As soon as you’ve paid the contractor, you no longer have the upper hand.
It’s crucial that you keep the payment schedule in line with the work schedule. The contract should establish a series of payments to be made when certain parts of the job are completed. For example, your contract could stipulate that you’ll pay in three equal installments, with the last payment made after the project is complete, and after you and your contractor agree the work is satisfactory.
Never put down more than 10% upfront; that’s too much cash to hand over before any work is complete. Your contractor should be able to get any necessary supplies on credit.