I have read somewhere that Creatine supplementation improves brain function (along with its many functions with athletic performance). I will try to look for the source.
A very low-carb Ketogenic Diet (like the early phases of Atkin's) will bring your body to Ketosis, which slows down cognitive function, along with other physiological effects such as bad breath. However, insulin spikes caused by high-GI carbohydrates like sugar and white bread tend to give you a "crash" and probably would not be the best way to go. Fruits, vegetables, sweet potatoes, and whole grains would be your best options for carbs.
Also some moderate cardio has been shown to help increase brain function by increasing the blood circulation to to brain.