It's rare to get a nice flat lie on the course I play so here's what I do to get good results from these awkward positions.

The key to these shots is to keep the lower body quiet and use the hands more in the shot. By keeping your lower body quiet, it's easier to maintain the spine angle so you don't hit the ball fat or thin. You'll reduce weight transfer so the power will come more from the hands. Remember to swing easy though to ensure a crisp contact and maybe take one more club to encourage this and compensate for the reduced weight transfer.

With the ball above your feet you'll need to stand more upright to compensate for the slope. With the ball below your feet you'll need to lean over a little more and possibly increase your knee-flex, depending on the severity of the slope, to prevent yourself from falling over. Personally I also move the ball back a little in my stance to promote more of a punch shot.