Along with your approach, focusing on building a strong business relationship is also important. Do everything from your side that your client can expect from a reliable web design company. Be it sending monthly reports to them or updating the clients with plans of monthly support, to build a steady business relationship, you need to keep them engaged as well as interested.
Learn to Avoid Crappy Clients
If you have a bad experience in dealing with a nagging client, make sure that you avoid them. Those clients that refuse to pay even after getting quality service from your designing company, its better to avoid future contact with them.