B vitamins!

B vitamins, especially B12, folate, and B6 (pyridoxine) are crucial for normal brain function. A Swedish study tested the memories of 167 older people free of dementia or depression but who were at greater risk for Alzheimer's disease. On the morning they tested memory they also drew blood and tested for B vitamins. The people who had normal levels of vitamin B12 did better on their memory tests than those with lower levels of B12. Folate also seemed to help memory but not as much as B12.
Getting adequate vitamin B12 may help the brain stay healthy and help you retain your memory as you get older. Milk is a good source of B12. Persons who don't eat dairy or meat may be low in B12 levels unless they are taking a supplement. As a person get older, B12 is not absorbed as well, so a lack of B12 is common even in non-vegetarians. The Institute of Medicine recommends persons over 50 get most of their B12 from fortified foods or a supplement because they are better absorbed. Be sure you get plenty of B12 daily. Good sources of folate include green leafy vegetables, legumes (especially lentils), oranges, and food yeast (add to tofu, soups, roasts).