When you advance the ball with a dribble at a constant rate of speed, it is relatively easy for a defensive player to guard you. He/she simply matches your speed, and stays with you. Make the job more difficult for the defender. Use a change of pace.
The change of pace involves varying the rate speed with which you advance the ball. As soon as the defender matches your speed, slow down quickly. As soon as he/she slows down, speed up quickly, and so on.
change-of-pace dribble, like the stop-and-go (see my tip, the Stop and Go Dribble), are excellent ways to deal with a defender who is guarding you closely and aggressively.