The above article (from the Journal of Strength and Conditioning) discusses some of the proven supplements for increasing performance in endurance athletes.

Some key points from the article:
– Endurance athletes typically sweat 1.2-1.7 liters per hour of intense exercise, a drastic reduction in performance can be measured with only 2% dehydration, so hydration is key!
-Electrolye-Carb Drinks – Most have 50-110mg sodium per 8oz.  Endurance athletes need to consume at least 600mL of fluid per hour of exercise, minimally.
– Carbs – During moderate intensity exercise (65-70% max intensity) the average athlete uses 60 grams of carbs per hour.  Carbohydrate stores can be severely depleted after 60-90 minutes of exercise, so anyone competing in an endurance-event that lasts longer than an hour should consider a carb supplement.  It is recommended that an endurance athlete consume 30-60 grams of carbs per hour (not fructose as it leads to decrease in performance)!  A  concentration of 6–8% carbohydrate drink (e.g., 6–8 grams of carbohydrate for every 100 mL of fluid) seems to be most effective.  Adding small-moderate amounts of protein (15-30grams) to the carb beverages, especially directly post-workout has shown to be even more effective in decreasing recovery time.
– In a study with 2 groups of cyclists, 1 consuming a carb-drink every 15 minutes and the other only drinking water, the group consuming the carb drink extended their average time-to-fatigue by 47 minutes over the other group!
-Caffeine – One of the most effective supplements for endurance athletes, caffeine has been shown to reduce fatigue, increase energy, increase the use of fats as a fuel source, and increase total amount of calories burned.  Even a dose of 195mg showed a 15% increase in work capacity with cyclists.
*As always, one should consult a physician before beginning a supplement or exercise regimen.