Originally Posted by Village Genius

First off, don't work for free. If you are going to do so leave no misconceptions that they stand in your debt for this and they can get fussy after they pay you. Go as far as to offer a complete refund of $0.00 for your services. Walk away the second they get annoying; you certainly have better things to do than waste your time with an annoying freeloader.

When doing it for money, charge by the hour (see my article on the subject here). Have a detailed outline of what you are going to and what you think its going to cost, make it clear that new things take time and therefore you will charge more. You will see a lot less scope creep after you've done that. Have it in a written contract as well, they take those a whole lot more seriously than emails.