Surpisingly enough, hydration has a lot to do with one's attention span.
Make sure you are getting enough water.
Hopefully the lecturer will be humane and give breaks. During the breaks be sure to stretch and move around.
My own attention span is rather limited, and I have not found stimulants or supplements to be much of any help, and, frankly, various imaginative and mental techniques have only been so helpful in having me remain attentive for long periods of time. So, I'd bring a tape recorder and learn to take better notes.
Sometimes enagaging your imagination in the lecture material helps, making jokes about it, argumentative comments or questions in your notes (that is, disagreeing actively to what is being said), or involving yourself with the info in some imaginative manner (if the lecture is about WWII, then imagine yourself as a soldier in the situation described, or a spy, etc.).
I'd try this and try that and see what fits. But definitely make sure you're getting enough water before, during, and after the lecture, and invest in a good recorder.
You can get one where you can play the lecture at a faster speed.
Galantamine is supposedly a good supplement, but after using it over time I am not incredibly impressed with the results.
