In watching warm-ups, and in particular throwing warm-ups, the main problem I spot in players trying to make a team is their throwing motion. Granted, some of the players had been taught the correct form for throwing the ball, but a lot had absolutely horrid form.
Here are a few drills to fine-tune your throwing form.
Good Mechanics
The bringing back of the throwing hand is done in a graceful, arching movement. The hand swings back and up, with the ball facing backward at the top of the arch. This can be practiced by having someone standing about five feet directly behind you.
Take a ball and from the hand in glove position--as if the ball were just fielded--separate the hands with the ball hand swinging first down, then back and up until your throwing arm bicep is parallel with the ground and you have a 90-degree angle with your forearm pointing up and the ball facing backwards.
Looking at this motion from the third base side (right handed thrower) the ball hand starts at nine o'clock, swings down to six o'clock then up to 12 o'clock. If you release the ball at about three o'clock (for the drill) and your form is correct, the ball should be easily caught by the person standing behind you.
If the ball goes to the left or right of the catcher, you are not using a proper path for your arm. Practice this drill until you have the proper arm motion, then, instead of releasing the ball, go into your throwing motion, striding with your lead foot and following through.
Load n' Go
If you need additional work on this motion, try the load and go drill. Stand with your weight evenly distributed between both feet and in the position of already haven taken your stride, with hips facing towards third base (right handed thrower).
Transfer your weight to the rear foot and lift the front foot off the ground. As you come forward with the ball in the throwing motion, transfer your weight back to your lead foot and make a throw.
If you work on this drill, it will improve your accuracy and distance--and make sure you have correct throwing mechanics.