Years ago (see below) I thought that I had the same problem as reported by many
freelancers: So-called colleagues, customers or social networking acquaintances asking for:
something that will require time, efforts and my expertise - and that is usually paid (well).
The problem has vanished or did never really exist for me. As long as you do not believe you need to make everybody happy you can simply say no to such not-willing-to-pay pseudo-clients. And as long as you are selling some service that somebody (desperately) needs (who is willing to pay).
But it is hard to say no to the paying clients. Especially if they are appealing to your vanity and to your reputation as the Number One / Guru / Diva.
Realistically it is not so much the fact that I can solve a problem in such a marvelous way. But I am probably the only available person. Or the only person that is as crazy as considering to work under such conditions - in terms of risk and schedule.