To teach [tag]teamwork in baseball[/tag] to my little league team, I have had great success working on defense using a “three (or more) lines” idea.
For example-we might work on picking a runner off of third base. We will have a line for catcher, a line for third base, and a line for left fielders.
The [tag]baseball coach[/tag] will toss a ball to the catcher who would come up throwing to third. Meanwhile the left fielder is immediately moving to back up the throw.
After the play is completed we rotate. The catcher hustles to left field, left field hustles to the back of the third base line, and the third baseman hustles to the back of the catching line. Each player keeps moving and the idea of backing up is heavily emphasized.
The basic idea of working in lines of three or more and emphasizing backing up has helped us a lot and can be used to teach/drill many situations.