I've been unconsciously practicing number #13 since the beginning of our start-up. Every time I close a deal I'm thinking, "it's not over until we start getting paid from these guys."
That attitude has served me well, I mean, I've had groups back out on us after they signed an agreement. What are we going to do? Lawyers cost to much and who wants to work with someone who doesn't want to work with us? Our customers are our partners and we love them--but if the feeling's not mutual I'd rather cut it off.
It's funny, I'm so optimistic about our success--I'm an entrepreneur after all--but I'm always waiting for the other shoe to drop when closing a new customer.
BTW: I should mention that our customers are newspapers, who are notorious for taking a long time to make bad decisions . . . so when we close someone, it's time to break out the scotch.