If you get the requirements in writing, and add a clause to your contract that says they have to "bring money" if they want a change, and sacrifice their retainer as liquidated damages, they think more up front. I haven't had anyone change their requirements in over 10 years, once they're on paper and signed off.

great when you can say no

The typical "wage slave" can not do so.

when I did/do consulting I do, largely, do like you; however, while I, like you request a written spec and state "additional charge if the spec changes" I still expect, and see, changes.

This ia an assumption, based on small snippets from your posts, but is it not so that your typical customer is the end user?

Erik