And one of the big picture items to keep track of is your sleep. It is sad, but one of the first things that goes when our mental functioning is impaired is our ability to judge how impaired we are. Which means that when you impair yourself, for instance by sleeping too little, you have no real monitor of how badly impaired you are. And the effects of lack of sleep are pretty serious, particularly for programmers. If you're noticing it, you need more sleep, then trust me, you would benefit a lot from having it.
In addition to tilly's very valid points about sleep (I just spent the better part of the last two days getting caught up myself), I'd also like to point out the importance of doing something, anything - walking, running, biking, swimming, whatever works - to get your body moving and blood flowing to your brain. You don't have to be a world-class athelete. You just have to do it. The benefits are more than worth it, and not just to your mental concentration.
Now, where're my biking shoes?