Drink coffee. Everything from test scores on college exams to chess games have been shown to improve after a cup of coffee or other drink containing caffeine. It is a temporary effect, however, and caffeine may have adverse long-term effects for regular users.
Avoid Sugar. Actually any simple carbohydrates taken in large quantities can give you that sluggish feeling that makes it difficult to think. This is due to the insulin that is dumped into your veins after the sugar is. Don't eat white flour, sugar, potatoes, and other carbs before an important meeting, or anytime you need full use of your intelligence.