Here is an interesting article about how to optimize your intake of caffeine. Some highlights:
small hourly doses of caffeine (.3mg per kg of body weight [approx 20 mg per hour; thanks digg!]) can support extended wakefulness
Caffeine may increase the speed with which you work, may decrease attentional lapses, and may even benefit recall – but is less likely to benefit more complex cognitive functions, and may even hurt others
The beneficial effects of caffeine may be most pronounced in conjunction with sugar.
How do you consume your caffeine throughout the day?