The best time to brush is normally right after breakfast and before you go to bed. However, if you eat sugary, sticky food or drink sweet coffee, tea, sodas, wine, or smoke during the day, then after lunch would be better, though it means bringing your teeth-cleaning kit to work.
Using a mouthwash that is more than a breath freshener will also help, such as over-the-counter ones with the ingredient cetyl pyridinium, or ask your hygienist for a recommendation for your case.