Take a shower, take a walk, watch some TV- don't focus for the moment. Let your brain wander- it's trying to integrate what you're learning, and work through something. Brains can't think all of the time- they need down time to sift through new stuff. Try to get some sleep...that part is hard, but it's really better for learning. Drink lots of water, take lecithin, ginko, fish oil. Chew mint gum- it helps keep your brain focused. Use yellow legal pads- the color is easier on the eyes. Don't highlight, paraphrase and make notes instead. Under no circumstances should you attend your best friend's 21st birthday party the night before your 8:00 am botany final- especially if you have to get someone to drive you to the test because you're still rock star drunk

. You'll probably still blow the curve, but boy that's just not fun, and somewhat embarassing.
Your peak retention time is 10 minutes before the test- I used to condense-down my notes from the semester, and then arrive to the test about 20 minutes early and review them in the hallway (after I learned the error of my ways).
Good luck! You'll do great.