The best one is, "There's only $x left to do. Give me a cheque for $X (ususally 90% + of the balance) and we'll get the rest done right away.... with so little left to do, I need to see you come through with the payment so I know I can trust you."

My answer is, if there is that little left to do, come and do it so I don't have to write two cheques.

As soon as you pay over 64% of the total contract value, you do not have the upper hand anymore. Money talks and bullshit walks.