Try to keep 4 or 5 balls in the same spot just outside off stump, maybe even give them all a touch of late swing away from the batsman, then pitch the last ball up outside off stump jagging in and swing it in viciously. If it's wrong it's a boundary ball, if they play it sensibly its a dot ball, if it's the perfect delivery it swings at pace, hits the deck and jags some more, and the off stump looks lovely as it flies into the keepers midfift :P