I offer a 12.5% discount for customers who use 'express service' which basically means that they agree to pay within 7 days of invoice and agree to use my simple NDA/contract and not use up my time doing contract negotiation.

If they don't pay on time, they lose the discount.

I've been lucky: only one no-pay in 12 years, and that was for about an hour of work on what was originally a half day estimate (the "customer" sincerely thanked, but said that it was not worth his trouble to pay a tiny one hour invoice). I just let it slide, but in retrospect, I wish that I had re-submitted the invoice.