An off the wall answer...hope it helps:
Though I drink them daily, I would stay away from caffeine if you're not used to it...the come-down is a bitch. Believe it or not, apples have a better "pick-me-up" effect, but make sure to eat the skins as well.
If you are on your feet so much, invest in some good "gel" type insoles with a good instep (as long as you don't have flat feet). Believe it or not, they help your posture and vitality, accuteness, awakedness....whatever you want to call it.
I used to work in retail and have done way too many trade shows. I swear by my Doc Martens and insoles and kill the caffeine when I can't have a constant flow.