Examples:

• I'll be on the ball, but give me a little leeway.
• I need to be on the ball all the time.
• You want me to be on the ball, don't you?
• You gotta be on the ball.
• They seem to be on the ball.
• He seems to be on the ball.
• So when he suddenly had an idea, you had to be on the ball and take notes.
• You need to be on the ball to deal with customers face-to-face.