With all that being said, Thomas, bear in mind there are times when it is all right, and encouraged, to do a shoot for free. For instance if you are trying out something entirely new (techniques, lenses, etc) or perhaps even for charity. If you pick up a lensbaby or a lens you’re unfamiliar with, don’t let the paid shoot be the first time you use it in a professional setting. Grab a friend and go to town, delivering the same kind of quality you would a client.
As far as charity goes; we had a bad fire here in Colorado last summer and many families lost their homes. That means a lot of family photos went with that. Many local photographers were offering either extreme discounts or free shoots to the families affected provided they booked within a certain time frame. That kind of publicity is good but it doesn’t give clients some false expectation that you do that all the time.
Overall, I want to reiterate what everyone has been saying: Charge what you’re worth. Your service has high value and you deserve to make money for said service. Don’t sell yourself short.