As a better player, solid ball-striking generally isn't a problem for you. Your body instinctively knows how to get the center of the clubface on the ball. That's what makes hitting from downhill lies so frustrating: Your natural instincts don't automatically kick in and ensure solid contact. The downhill lie is a difficult situation that confounds even the best players, but a few rules of thumb will help your consistency.
BALL BACK IN STANCE
To compensate for the downhill slope, you must make a steep downswing into the hitting area. On a moderate downslope, play the ball about three inches farther back in your stance than normal; this encourages the steep downswing angle you need. But be aware that this also delofts the clubface; for example, it may turn a 7-iron into a 5-iron.
DON'T HIT LESS CLUB
Despite the reduced loft, stick with the club you would ordinarily hit. There are a few reasons why. First, the downhill lie is best handled with a three-quarter swing to promote the descending blow. Second, it's a good idea to choke up on the grip for added control. And third, it's important to swing easy, since the hill magnifies any deviation from a square clubface and an easier swing provides your best chance of squaring the face at impact. Add these things together and you've compensated for the reduced loft.
BACKSWING: STAY COMPACT
There's less margin for error on a downhill lie, so it's important not to move your center of gravity much during the backswing. Anchor your weight on the left leg at address, match your shoulder line to the angle of the downslope, and keep your weight right as you swing back. Take the club back with the arms, make an abbreviated turn with the shoulders, and feel as if you haven't moved much behind the ball. Your hands shouldn't reach much higher than the left shoulder and the clubshaft should be well short of parallel at the top.
DOWNSWING: SMOOTH EXTENSION
The downswing on a downhill lie must be as smooth and controlled as the backswing, even though the impulse is to let everything go and swing hard. If you fall victim to this, you'll lunge at the ball and invariably mishit it.
Here's a better plan: Without feeling like your entire body is drifting down the hill, let your weight transfer to the downhill leg and drop the clubhead into the back of the ball. Let gravity do most of the work; it's a good way to stay smooth. Through impact, focus on extending your arms as far as possible down the hill, pointing the club at the target as you follow through. The more you extend your arms without letting your upper body move down the hill, the longer you'll keep the clubface square to the target.