Play: This can't be stressed enough.  Besides kicking off the contest, the plate umpire must always point the ball back in play with runners on base when the pitcher is on the rubber with the ball and the batter is in the batters box.  This is essential. It allows the players and your partner to know when exactly a ball is back in play. Often a pitcher will throw over to first as soon as the umpire points the ball back in play.  If this is not done, is the ball back in play or not?  Should be accompanied by a verbal signal of  "Play!" 

Ball: No physical signal.  Plate umpire gives a verbal signal of "Ball" loud enough to be heard by at least the batter and catcher.  Do not indicate location of the pitch.  "Ball" will suffice. 

Swinging Strike: No verbal signal.  Plate umpire gives his strike mechanic only. Made by the right hand and arm.

Called Strike: Verbal and physical signal.  Your best banger for called strike 3.

Dropped Third Strike: Technical mechanic can vary here.  I usually signal a strike and keep my arm up to indicate that the play is not over.  If there is a question as to weather or not the batter swung, state "Yes, he did" to indicate that the batter did indeed swing.  If there is a potential checked swing, go immediately to your partner for a ruling before being asked.

Checked / Half Swing: Point to the batter with your left hand, then come up with your strike mechanic and state "Yes he did" or if he didn’t swing, simply  state "Ball, no he didn't go."

Checked / Half Swing Appeal: Step out from behind the plate, point to the appropriate partner with your left hand and ask loudly "Did he go" or "Joe, did he go?"  If your partner indicates yes, then say, "Then that's a strike" while giving your strike mechanic and give the revised count.  The base umpire should never respond unless asked by the plate umpire for a ruling.

Foul Tip: Slide the right hand off the left hand in an upward motion and then come up with a strike mechanic.  Some umpires slide their hand 2 or 3 times then come up with the strike mechanic. This is usually a swinging strike and always a live ball, so no verbal signal is needed.

Time: Both arms raised above shoulder level, palms facing out.  Verbal mechanic is "Time" not "Dead Ball".  This should also be utilized when a batter has been hit by the pitch.

Foul Ball: Same physical mechanic as "Time".  Verbalization is "Foul" not "Foul Ball."

Safe: Lift your arms from your side or knees, fingers together and extended and lift them chest-high.  Then come straight across to the sides of your body while stating "Safe."  No fly-away bird mechanics needed.

Safe, missed tag: Same physical and verbal mechanic as Safe.  Followed by both arms coming together in front of the body and then sweeping to one side while saying "He's off the bag."

Out: Right fist together covered by the thumb, lift your arm crisply up above your head level and state "He's out."  Use your best banger for a close one.  Work on it in front of a mirror.

Out on the Tag: While stepping towards the play point to the tag with the left hand and say "On the tag.  He’s out."

Fair Ball: There is never a verbalization for a fair ball.  The umpire simply points into fair territory with either his left or right arm to indicate that the ball is fair.

Foul Ball: Same physical mechanic as "Time," with the hands up a bit higher and then point to foul territory. The verbalization is "Foul",  not "Foul Ball."  Not needed for an obviously foul ball that everyone already knows is foul.

No Pitch: Call "Time" or "Time - No pitch" if there is a question as to pitch validity if the pitch was delivered.

Count: Hands up around eye level with the appropriate number of fingers extended.  Proper mechanic is to state "3 and 2" or "3 balls, 2 strikes", not a fist and not "Full count."

Run Scores: Used for a time play when there is a question as to weather or not the run counts.  Point to the plate and then the press box or scorer and state " The run scores, score that run" as you do so.

Run Does Not Score: Also used for a time play when there is a question as to weather or not the run counts.  Point to the the press box or scorer and state "No run scores, no run scores" while raising your arms above your head in a sweeping "X" motion.

Base Awards: The appropriate umpire points at the runner to be awarded and states "You - 2nd-base", or You - Score" while continuing his gesture toward the base being awarded.  When multiple runners are to be given awards, the umpire should begin with the runner closest to home plate, and work back from there.

Ground Rule Double: The signal is the right arm with first two fingers extended overhead.  Keep in  mind that the ball has left the field and is dead, even if it should bound back onto the playing area.

Fan Interference: Grabbing of the left arm at wrist level with the right hand.  Place runners.

Home Run: The signal is the right arm with index finger extended overhead in a circular motion.   Once again,  the ball has left the field and is dead, even if it should bound back onto the playing area.

Balk: Point to the pitcher and call "Time! That's a balk."  A balk is an immediate dead ball in High School baseball.  Place runners.

Obstruction: Point to the obstruction when it occurs and state "That's obstruction."  Call time when the play has run its course and place the runners if needed when appropriate.

Interference: Point to the interference  when it occurs and state "That's interference."  Call time immediately or after the play and place the runners if needed when appropriate.

Catch / No Catch: Out mechanic with verbal "That's a catch" or Safe mechanic with verbal "No catch - No catch."

Infield Fly:  Point into the air with your right arm fully extended.  State either "Infield fly - batters out" or "Infield fly if fair — batter's out" if there is a fair-foul possibility.