get to the ball early so it's at shoulder height, don't let it drop.
hit down into the court with a flat shot if the ball is high.
if you let the ball drop, use top spin and brush up on the ball to bring it down into the court.
I usually attack the backhand side a few times in a row, then switch to the forehand to get them moving the wrong way.
you can disguise a down the line forehand by making it look like your are hitting an off forehand, then at the last moment flicking the ball down the line.
practice, practice, practice.
get a coach to drill you