One of the most difficult plays in baseball is the fly ball or line drive that is hit directly into the sun.
It's important to get a good jump on the ball and quickly anticipate where you think it will land. Often the ball is lost soon after being hit, which means your first analysis of where the ball is going to land is essential.
Once you have lost the ball in the sun, continue in the direction of where you believe it will land. Listen for other outfielders to tell you if you're going in the wrong direction.
When you turn to find the ball, either while running or after reaching the spot you think it will land, use your glove to shield the sun from your eyes. It's important that you don't stare into the sun directly. If running at an angle that makes it difficult to get the glove up, use your throwing hand to shield the sun.
As your running, if the ball is above the sun, shield your eyes by looking over the top of your glove towards the ball.
Once the ball has dropped into the sun, switch to shielding your eyes with the bottom of your glove. You will be looking under your glove for the ball to drop out of the sun. Many players make the mistake of continuing to look over the top of the glove for the ball. This is incorrect since the ball, as it is coming down, will be visible first below the glove. If you look over the top of the glove, you may never see it.