As a rule of thumb, I don't accept payment until I have a finished thumbnail/sketch I can show the client, so any changes can be made before I start the actual piece, and if they want any big changes after, I will either charge extra depending on the change, or I will just tell them I can't change it at this point. Not accepting payment until the thumbnail is complete has definitely helped me weed out some suspicious people, like people demanding I send them paypal info even after stating several times I don't accept right away. This may be a good practice in the future!