i drank alot of coke, water, and had sandwiches.
weird thing, while i was in my math based classes, i used to put on the headphones and blast the hell out of them with music, so loud that i was able to zone out of the music and acutally focus on the actual material. i dont know how, but it worked for me.
if its reading based, i get in a comfortable position, everything quiet around me, and just start to read. take breaks w/ streching and eating some snacks.