Here's a great drill to practice proper pass breakup technique.
Setup
You'll need one quarterback, one receiver, and one defensive back. The receiver will run perpendicular to the quarterback at 10-15 yards in front of him. The defensive back will trail directly behind the receiver.
This drill should be done at 50% speed. As the quarterback throws the ball the defensive back moves into the proper position, and knocks down the pass with the front side hand. Always be careful not to swat at the ball. Anytime you break up a pass, you want to treat your front side hand like a baseball bat, and you're bunting. Don't swat at it, just get your hand on it.
Purpose
The purpose of this drill is to engrain the proper technique of breaking up passes into defensive backs and linebackers. The biggest tendency is to swat at the ball, which usually results in a miss.
Variations
As a variation to the same drill, have the defensive back break on the ball and intercept the pass.