Lots of good suggestions here.

Here is another practice that I have found useful. We all know that the project (and spec) will change after the customer starts playing with the first version. That is why we quote a fixed price plus an hourly rate for "Future enhancements."

Draft a document that says you have completed the project to the original spec. Have the customer sign it before you make any future revisions. That helps make it clear that the changes are not included in the original project and will result in extra charges.

-- Tom