I used to send customers .wmv files of the end product for their final approval, but always got a problem with the fact that they would send it around every office so that everybody at the business would be able to give their 2 cents. Of course, everybody wanted to impress the boss with their input, and the changes started to get totally unreasonable.
Nowadays, I invite the client to come and sit in the edit suite with me, and view the finished product there. They are always impressed by the atmosphere, technology etc, and are more complient to your advice and explanations. They have ONE chance to make any changes, and one chance only. Thats for free, and written in the contract.
After the master is delivered, any changes are billed at a normal daily rate.
Simon