Late and unpaid bills can hurt your cash flow. Assign someone to track your billing. Then put a process in place for if a bill goes unpaid. That can be issuing a second invoice, making a phone call and even levying penalties such as extra fees at certain deadlines.
Make a plan for if clients are 30, 60 and 90 days late. Remember, every late payment is an interest-free loan that hurts your cash flow.