I used to do 50% on start date, 50% invoiced by mail at completion. But that turned into a cash flow nightmare.
Now, we do 20% to hold their place in line, 50% the day we begin, 30% when we finish. The better cash flow seems to make the night sweats go away.
I try to do a walkthrough the day we finish. Most (about 55%) pay the balance during the walkthrough, making the final invoice unneccesary.