Run towards your defender, concentrate on his chest and head straight for it.
Watch his feet and body to determine where his momentum is carrying him. Draw a visualized line across the field that shows which direction he is traveling and use this to set up your juke or dodge.
Twitch your shoulders and hips away from this imaginary line in the direction you want the defender to think you are going. Convince him that you are going this direction with an additional head fake, or body lean. The point is to get the defender to commit and change his initial momentum. He will try and adjust for a new point of contact (where his body will contact yours), usually a few feet ahead of you in the direction he thinks you are now going.
Wait a split second, until the defender lowers his head for the tackle, and then stop suddenly, plant your feet and spin or juke and cut across the field in another direction. The defender should fly past you, because he is expecting to contact you at the point you established with your first move .