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.