We find that sending a copy of the outstanding invoice, supporting e-mails and a letter advising them that if you don't get payment within 7 days you'll start court proceedings to reclaim the debt usually works quite well. We don't always get the full amount, but in most cases it prompts some action from the other party and if you get 75% of the amount outstanding its easier than actually going through the court. As someone already said, time is money, and going through court will take plenty of your time!
If you do get a part payment but you still aren't happy - make sure you pay in the cheque and wait until it clears before going any further. Cashing the cheque does not indicate you are happy with the payment and the issue is settled, you can still take them to court for the rest!!
If you've got a written agreement (e-mail will do) and you've kept your side of the agreement completely - you should win at court, and you should get awarded costs and interest on the debt too!!