To make sure I get out of the office at a reasonable time every day I book activities with other people, thus making sure I can't cancel, unless it's urgent of course. It just gives you another level of commitment so that you don't end up staying to "try one more thing" at work! 
So for example, I like to book in for a tennis match in the evenings. In the wintertime, I tend to book classes in advance like yoga, that way I don't want to waste my money by not turning up. 
