to get them to write down what they want, and sign it ...

If they change something you have a bit of heartburn, a few late nights, maybe an extended schedule, and a few extra bucks, usually on the order of 20% or so if the change is small. If it's 20%, that means it took less than an hour to implement the changes and fewer than 8 hours to document them.

Once your clients realize that changes cost money, they think about what they ask you to do before they sign, instead of after.

It costs them a lot more if you have twice the speed, resources, power, etc, and you don't get the follow-on work, because someone else tells them, "I could have designed a circuit that costs you less..."

RE