I get up and walk around. I drink plenty of water and so that I have to go to the bathroom frequently.
Snacking helps too. I try to bring lots of fresh fruit with me so I can snack all day long and not feel guilty.
I also planned my matetnity leave already and have a ton of work to finish before I go. Deadlines keep me awake, too.
Try eating trail mix. Gives you lots of energy!!