[5 sport ref]

I agree with Striker991 that timing is very important. Slow down and slow down some more. A little trick I was taught to improve timinf when I was starting is let the catcher catch the ball and think to yourself "it is a" then call ball or strike. Some other things I have learned along the way: Call what you see and at lower levels expand the zone left to right. Coaches can tell if you expand the zone up and down but not left to right. A ball a few inches inside or outside call a strike at the lower levels, make them swing and be consistent. What is a strike in the first inning better be a strike in the sixth or seventh. Remember, you get paid by the game not by the hour and coaches like strikes and outs. Control the game. Let the play finish then make the call. On tag plays ask to see the ball to be sure the fielder has control. Anyone can call ball/strike, fair/foul and safe/out which is about 98% of the game, where you make a name for yourself is on the other 2% of the plays so know the rules and be able o come up with a ruling quickly.

Hope that helps.