>What about 1/3 up front, 1/3 when I deliver for testing, and 1/3 when they accept?
>

50% up front.
40% as progress payments. (several payments at milestones if the project has any size)
10% or less as acceptance.

2 weeks with no bug reports = acceptance.

About the kill switch - if you make sure you are always working with their money, it is not needed so much.

If you leave a large payment hinging on "acceptance" they will hang you out to dry.
Even if they fully intend to pay you, it is just human nature to get very demanding in that situation.

If they have paid almost all the money and there is just a trivial amount due for acceptance, their motivation is far more likely to be to help you get it done and delivered.

Good Luck!