Oh, easy: I just remember the nasty rash that my nurslings will break out in if I eat the wrong things. :-/
Our office has a "Sunshine Club," where you pay $2/month and then get a card and a cake on your birthday. Meanwhile, you get to join in the celebration of everyone else's birthdays. I don't belong, because I can't eat the cake, and there's just no way I'm going to put anyone through trying to find me a gluten-free, dairy-free, corn-free, soy-free, egg-free, sorghum-free, canola-free cake for *my* birthday.
The bakery they get them from is fabulous, though, I've heard. It's across the street from DH's work, so I have some good info on it.
I bring a ton of food with me to work every day, too. That helps. (And I bring my own treats... cookies or candy or whatever... and keep a bag of good turbinado sugar for my coffee in my desk.)