Take the pressure off.
Deal with pressure and expectations by realistically assessing how you think you'll go, and working to do the best you can. If other people's expectations are pressuring you, talk to them and try to get them to back off. If you're putting too much pressure on yourself, try to realise failure isn't fatal. It’s likely that the worst that can happen is that you take it again, with a massive head-start from the work you've already done.
Do it together.
There are other people studying for the same exam, assignment, assessment and they probably don't like having to lock themselves away to study any more than you do. Get together with them and take the books outside from time to time. If you're not as strong in certain areas, it could help to talk to fellow students as well. If you are good at it, share the brain-wealth.
Go easy on the substances!
ALL OF THEM! – Caffeine, Cigarettes, Coffee, No-doz, Alcohol, Marijuana, Ritalin, Dexamphetamine and any other drugs. Any drug you think will help you study is actually a short-term fix that'll probably make you feel much worse and cause you to underperform later.