increasing arm strenght in outfield

Alot of long toss.

dad10 is right the best way to get better at throwing long is to throw long alot, that way your entire body is in condition to perform that throw and will respond when needed.

In addition to long toss assess your levels in the following:

1. Throwing Mechanics
2. Ability to generate force from within

One last thought. Your foot work when receiving the fly ball or ground ball. This can make the difference in the time it takes you to release the ball. note I said receive, that is because your intent is to throw the ball to another player. Knowing where to throw the ball and setting up for that throw will help reduce the amount of time you take in getting the ball to the next player.