The Agency Byte blog has a post on 'scope creep' - why it happens and what you can do about it.
"Scope creep begins at the moment when your client asks for something outside of the scope of work for which youâ€™ve contracted (you do have a contract, right?). Naturally, theyâ€™re asking for this thing to be included at no extra cost. Actually, theyâ€™re probably not asking. Theyâ€™re probably acting like itâ€™s always been understood that this thing will be included. Duh."
Defining Project Boundaries (and Keeping Your Client Within Them) [via Freelance Switch]