I also struggle with weight. I'm not overweight right now, but it is a constant vigil to make sure the pounds don't creap on...
1.) If you can, use your lunch break to walk. Get out of the break room and the constant parade of food. At night, I brought a portable DVD player and would play 10-minute workouts in an empty room. Or bring a MP3 player and march to the music.
2.) DO NOT DRINK!!! After work stress is awful, but avoid the wine, booze, beer.... You feel good and relaxed for a few hours, but the pounds will come on so fast.
3.) Find coworkers who also are trying to be fit and use their support.
4.) Learn to say "no" to the birthday cakes, party leftovers, conference leftovers, doughnuts, etc. It is hard, but this is a big place where people gain weight.
5.) Bring your lunch and snacks every day. This is a good way to control what you are putting into your body.
6.) Stay hydrated.