Which vitamins and supplements are recommended to take to improve concentration, sharpness and focus?

I feel quite lethargic at work and find it difficult to sharpen my mind and concentrate. I know small amounts of caffeine help the brain stay alert, but are there any supplements that would aid this? I have heard omega 3 and cod liver oil are good, but I'm not sure if this is true or what doses. I'm about to start a new job and I want to be on the ball, not daydreaming.