The most important principle in shielding is to keep your body between the ball and the opponent.  This can be done by dribbling the ball with the foot furthest from the opponent or by turning and swiveling to put your body between the opponent and the ball.

The biggest mistake when shielding is to have the ball between your legs.  You want the entire body between the defender and the ball.  Another common mistake is standing too straight when shielding.  Whey you shield you want to get low, almost as if you were sitting in a chair.  The easiest way to demonstrate this is to have a player place the ball on their outside foot and stand straight up.  Have a defender approach the player as close as they can and poke the ball away.  This will be very easy with the player standing straight up.  Now have the player exaggerate and crouch into a sitting position with the ball still on the outside of their foot.  Note how the defender will no longer be able to reach the ball to tackle it away.

Another factor when shielding is the position of the arms.  Although this is a common mistake made by referees, it is perfectly legal to shield with your arms as long as you do not push the player with your arms.  You arms should be held away from the body - there must be space between the arms and the body.  Using the stance described above, a player can easily hold off a defender using their arms.

Another means of shielding is a shoulder to shoulder charge.  A fair charge is defined as a shoulder-to-shoulder hit.  Referees in general will look at the player carrying the ball as if they were standing in the center of a clock face - the player is facing 12 o'clock.  An approach from anywhere within the clock except for the 4 o'clock to 8 o'clock positions will be considered legal (although 4 o'clock and 8 o'clock themselves are toeing the line).

Note that shielding is sometimes done even if you do not have possession of the ball.  It is very common for forwards to hold off defenders while waiting for a pass.  To do so, they must lean backwards into the defender with their arms out wide for balance and as a legal shield.

Once you've successfully screened, the next step is to continue advancement of the ball.  If a pass is an option then most certainly this is your best bet.  Often dribbling back to make space is the most appropriate maneuver.  Passing back to to a teammate is usually the best option though.

When screening the ball, only attempt to turn if the defender is 5 feet away or more or when the defender is not touching you.  Screening while receiving the ball is easier if the player can move into a position as tight as possible against his marker - the closer the players are the more difficult it is for the defender to see the ball or to tackle for it legally. You should lean back into the marker when the ball is within playing distance - not too soon or it is an obstruction foul.  If the defender moves to the side of the player to see the ball better, the player can 'spin off' him in the opposite direction. When turning, use a very strong inside arm to hold the defender off - the difference between a legal technique and an illegal practices is on a very thin line. Work the ball with the outside foot. · If a defender tries to come around you, you can beat them by turning in a tight circle using the outside of the foot. If they are approaching and attempting to pass from behind on your left, use the outside of the right foot to cut away to the right.