Barry Moltz points out this Entrepreneur.com article by Mark Diener about Mastering the Six Laws of Money.  Write these down and keep them in front of you.  A sampling:

1. Money sooner is better than money later. Eliminate the risk of not getting paid by getting your money upfront. This also tests whether the other side is serious.

2. Go to the source. Paul could wait until John gets cash from Peter, but if I were Paul, I'd rather call Peter myself. The names may be confusing, but the lesson's simple: Go upstream.

6. Get the right to offset. Savvy buyers won't pay everything when a deal closes. They insist on the right to hold back money to cover against future problems