Outrun your opponent. If you are smaller than the player you are defending, compensate by outworking your opponent. Get up and down the court faster. By beating your opponent downcourt, you can fight for position and force your taller opponent further away from the basket. You don't want the taller player to beat you under the basket, where the height differential will come into play.
Stay close to the taller player. By crowding your opponent, you can concentrate on harassing him while he dribbles. Stay as close as possible and go for steals if the player you are defending tries to maneuver around you.
Remain on the ground. Due to the difference in height, you should rarely go for a blocked shot. Trying to block the shot of a taller defender will lead to more fouls than blocks. Keep your feet on the court and remember that you'll enjoy more success going for steals.
Keep your taller opponent from getting rebounds by boxing him out. Due to the height difference, your opponent has the natural advantage when it comes to rebounds. However, if you are more aggressive and physical, you can outmuscle opponents for rebounds.