I can hear the "Oh, yeahs" from here.
That's why it is SO important to be specific about what a client should expect to begin with. Write it down. Literally. "Here are how many (ideas, scripts, designs) you'll get. Here are how many revisions you get. Here's the timeline - for BOTH of us. Here's what end product you will get. Here's how much it will cost."
If they quibble over the price/timing/hours, tell them exactly what they will lose by cutting it down. If they continuously change their minds, miss deadlines or ask for anything outside the realm of the original estimate, tell them how much more it will cost. Especially if you're doing a job for a small amount to get a good sample, explain this in detail. Tell them you're cutting your fee, explain you're doing it in exchange for getting something else out of it. Then, if they start nibbling it to death, you can explain why you'll need to charge them more. If you're not getting a sample out of it, then you need to get grocery money out of it.
There's no need to be an ass, just firm. Be reasonable in your pricing, realistic in your timing and deliverables, and honest up front. You'll both be happier in the long run.
