Healthy diet. Simple answer is eating a healthy balanced diet and exercise (physical and mental). If you happen to be deficient (or even borderline) in b12, thiamine, vitamin d, or vitamin e, so have a doctor check. Avoid smoking and keep alcohol to < 2 drinks/day maximum. Most any multivitamin is a decent insurance policy against dietary deficiencies. Ginkgo might help, but it thins blood like aspirin.
L-Glutamine. L-glutamine helps with memory and focus take1000mg at night.
L-Glutamine. L-glutamine lifts memory fog - start with 1000mg at night will help with sleep too. For people with irritable bowel can take upto 15gm per day.
Check vitamin levels. Sometimes vitamin b12, thiamine or vitamine e are low. Your doctor can check those levels.
Many. I would first start with a good multivitamin/mineral product. Choose one that is a truly natural, food based one. In addition, many agents help with memory: ginkgo, gotu kola, phosphatidylserine, acetyl-l-carnitine, essential fatty acids and others can be effective. Exercise is also beneficial, so get out and move it!