Red Bull is a good way to stay alert at the table, But I cannot stand the taste of it so for me it has to be Tea, Coffee, or Coke, anything that is strong with caffeine, to keep the old grey matter working. The only time I start to drink Beer or any Alcohol is when I make the Final Table and I am in the Money Then I reward myself for playing well.
Pro-Plus tablets can also keep you alert, as they are concentrated Caffeine and are available over the counter in any drug store, but they are not good for you they contain as much Caffeine as 50 cups of black Coffee.