Here’s a  good article about taking steps to get customer payments.

I would also suggest the following:

Be proactive by clearly communicating standard payment terms (i.e. net 30, due upon receipt) with new customers/clients.

Bill regularly so that customers don’t get hit with a bill for two months of service which may upset their monthly budget.

Follow up on late payments promptly…time is money.  The longer you wait to attempt to  collect; the less likely you will collect.