Six Scientifically Supported Ways to Crush Procrastination
Exercise also helps with focus. I find that a 30 min run before studying/exams help a lot.
Well rested sleep also helps.
Confidence by throughly practicing what you're being evaluating on really helps to. Not only will you be prepared, you'll be a lot calmer too when the time comes.
I know you don't like pharmaceuticals but they do help. Adderrall, wellbutrin, caffeine, ephedrine, all help, as they are stimulants. But use them sparingly and don't take them all at once, or they have the opposite effect. The jitters suck.