Hitting a golf ball when it is below your feet is an exercise in physics and swing adjustment for the golfer. It can be difficult to maintain your stance -- particularly when the ball is below your feet -- and that can lead to an awkward swing with poor results.
Bend your knees and hips more than usual when you address the ball on a sidehill lie.
Try to keep your weight back on your feet as much as you can. You will be leaning out over the ball and most of your weight will tend to go out toward your toes, so force your weight back.
Aim to the left of your target (for righthanders. Reverse for lefties). When the ball is below your feet your ball is going to veer off to the right at a relatively harsh angle. In order to compensate, aim left.
You'll need to concentrate on holding your form and keeping your balance, so slow your swing significantly. When your ball is beneath your feet, you can take a relatively full swing, but your ability to stay balanced will affect your swing's velocity.
Emphasize your follow-through when you are hitting a ball below your feet. If you don't finish the shot with your club at shoulder level, it will veer off to the right (for a right-handed golfer).
Tips & Warnings
Concentrate on form and not swing velocity when hitting a ball beneath your feet.