Brian Priebe was the Head Freshman Baseball Coach at Monte Vista High School in San Diego. He has written several articles on coaching baseball.  I really liked this segment he wrote on effective base coaches.  Enjoy!

Here are four characteristics of effective base coaches:

Communicate Clearly. Good base coaches are effective communicators. This must be true in spite of the fact that base coaches speak in "code" to maintain the element of surprise. Establish a language known only to your team. Use it consistently. Review your language and base running policies regularly with players during practice.

Have a corresponding set of hand signals. Verbal commands can be drowned out by noise from the stands or the defense. Hand signals are often more effective and discrete.

Ingrain the meaning of each verbal cue and hand signal during "chalk talk" sessions, scrimmages, and base running drills in practice.

Be Animated & Loud. Good base coaches are easily seen and heard by runners and batters. Base runners must be able to see the third base coach clearly in between glances at the fielders and the ball. Use exaggerated arm and hand gestures. For example, the third base coach can kneel or crouch down when instructing the runner to slide on a close play.

Make Snap Decisions. Be ready to react to the unexpected-- passed balls, wild pitches, catcher's rifling the ball to a base after the pitch, a bobbled ground ball in the outfield. Your brain must process information with computer-like quickness.

For example, when the batter launches a double into the gap with a runner on first base, the third base coach must consider all the following factors as part of his decision whether to send the runner home:

Base runner's foot speed,
Outfielder's arm strength,
Relay man's arm strength,
Score,
Inning,
Number of outs,
Which batter is on deck.

Here are two tips to improve your decision-making:
  a) Pre-pitch planning. Rehearse the possibilities in your mind before they happen. Anticipate the other team's moves. Warn the runner if you suspect a particular defensive play.
  b) When unexpected plays do occur, go with your gut. Have the courage to make the wrong call. You will find you are usually right when you follow your first instinct.

Encourage Aggressive Play. A good base coach realizes his capacity to set the tone for the offense. Players feed off the base coach's aggressive calls. When the coach shows confidence in the player's ability to execute, it boosts their confidence. Calling an aggressive base running game from the coach's box can spark a rally.

Take high percentage risks. When the game situation permits, force the defense to make a perfect play in order to tag the runner out. Pressure the defense into committing errors.

Some X's and O's:
The first base coach is responsible for all base runners from the time they exit the batter's box until they commit themselves to second base. Once a runner leaves first base, the third base coach assumes responsibility for the runner until he leaves third to score.

The third base coach must maneuver himself directly into the runner's line of sight. Remember to remain well into foul territory at all times and avoid making contact with any runner while the ball is in play.

On a base hit with runners at first and second, the third base coach must give verbal commands and hand signals to both runners. If the coach sends the first runner home, he then must pick up the trail runner and tell him whether to stay at second base or advance to third.

Who is responsible to coach runners once they leave third base and try to score? The on-deck batter. Base coaches should teach all offensive players the basic principles described above so that, as on-deck batters, they can stand behind the plate and help their teammates score safely.

Effective base coaches can influence the outcome of many plays throughout a game. Though their contributions are not always apparent, base coaches are critical to a team's ability to generate runs.