One of the most commons pitfalls seen by infielders is the mistake of fielding a ground ball too close to their body - often letting the ball get all the way in between their feet. This cuts down on their reaction time, and most bobbles tend to end up behind the fielder, making it difficult to recover and record an out on even a slow baserunner.
To help prevent your players from holding their arms too close to their body when fielding grounders, give them a mouthful of their cap! To perform the drill, the fielder should hold his hat in his mouth by the bill - the back of the hat should be facing away from his body. A coach then hits grounders at him, stressing proper footwork and fundamentals.
Since the hat will block a large portion of the player's field of vision, he will be forced to place his hands further out in front of him in an effort to see the ball into his hands - and thus, to field the ball properly. Though the drill naturally helps to ensure this (and most players do so without even being told), make sure that your players actually are reaching out, and not trying to simply field the ball blind, else you will do more harm than good!
Younger players should simply field the ball. Older, more skilled players can incorporate the hat in the mouth drill into their normal fielding routines - making throws to first, covering bases, even turning double plays.