Simple. Proper nutrition, hydration, rest and exercise are the foundations og good brain function. Artificial aids such as stimulants, herbs and such may help marginally in the short term, but are generally harmful or have never been proven to work.
Good night's sleep. The very best way to boost your brainpower and memory is to be well-rested. Getting a full 8-9 hours of sleep each night will do you far more good than trying to cram in an extra hour or two of studying when you're too tired to remember anything anyway.