Social pressure is a pretty big motivator for most people. I find that customers are the big driver, if they're willing to wait (accept delays, don't bring the deadline in when it's been agreed) then I really feel no pressure at all to work overtime.
When the customer is screaming down the phone at your grand-boss that you've failed to deliver and they have very little context for that and ask you if you can work extra to make them happy you're in an awkward position.
