Creativity is such an elusive thing. But if you have to 'be creative' as a fundamental part of your 'every day of the week' job, and you're paying attention to the periods when you're most productive (or completely unproductive), you're sure to have noticed a few things about the way you work.   I found this out the hard way very early in my career as an advertising copywriter. Come mid- to late-afternoon, I could…