Get plenty of rest the night before your lecture. This is one of the best ways to stay awake during a lecture or a long class. Since you should have a good idea of how long the lecture will be, prepare yourself ahead of time.
Sit in the front of the lecture hall. You are less likely to fall asleep while sitting directly in front of the lecturer. Plus in the front, everything is louder and brighter.
Participate. For any opportunity that the lecturer gives their audience to participate, volunteer your answers.
Take notes. Not only will taking notes come in handy as far as retaining information and being able to revisit the lecture ideas later, but it keeps you occupied and listening. Focus on the task and the lecture will be over before you know it.
Eat sour candy. This surge of tartness is sure to keep you alert. Even hot balls or some sort of cinnamon candy will work. Keep enough with you to last throughout the entire lecture if need be.
Drink something. Keeping something to drink near you will keep your hands, arms, and mouth busy.
Organize yourself. Use your organizer and day dream about the things that you need to get in order. Of course you won't be paying too much attention to the lecturer but you're at least awake.
Find something else to do. Prior to your lecture, stash something quiet that will occupy your time in the lecture--a journal, personal digital assistant, cell phone, etc.