Concentration: Maintaining Focus
I found this 'little gem' in a yoga book recently, which involves focusing on the tip of your nose for about five seconds, then on a specific point on the horizon for about five seconds.
To minimize risk in a game situation, I would suggest that this could be carried out when a ball goes out far behind the opposition's goal, allowing just enough time to prepare for the next phase of play. Alternatively, the keeper could do this from a dead ball (goal kick), or even as his own team is preparing to take a corner.
This exercise can actually provide keepers with a slightly different visual perspective; allowing them to adjust their stance or position accordingly in preparation for the next phase of play.
I would love to hear other keepers' or keeper coaches' tips for ensuring a high level of concentration and alertness during a game.