#1) Make a good backswing.
A good backswing will definitely help lead to a good downswing. Sometimes, when a golfer has problems with their backswing, it's almost impossible not to have problems in the downswing. It's kind of like one problem leading to another or the golfer has to make compensations to counterbalance the problem(s) that occured in the backswing.
#2) Maintain your posture.
So many times I see golfers come out of their posture on the dowswing and through impact. I like to tell golfers to stay in their posture and their angles until about halfway into their follow through. Then they can stand up at that point and face their target in 2nd half of the follow through. Changing your height either way in the downswing is not good. Dipping or standing up too early or at any point in the downswing will lead to inconsistency.
#3) Hold the lag.
Lag is basically the angle that is formed with the right forearm and the clubshaft in the swing. Hopefully this angle is correct at the top of the backswing if you've made a good wrist set or hinge. Then, in the downswing, simply try to maintain that angle as long as possible so that at impact the hands are ahead of the clubhead as you hit the ball and the clubshaft is leaning towards the target.
There really shouldn't be any wrist flip or unhinge in the downswing and a very minimal amount, if any, in the follow through. Once the wrists set in the backswing, that angle should be maintained as long as possible the rest of the swing. In the follow through, the forearms rotate and the right arm crosses over the left, but there shouldn't be too much re-hinge of the wrists. Keep the wrists firm and the hands out of the shot as much as possible. Golfers who flip their wrists in the follow through are usually not turning through and facing the target correctly.
#4) Swing down the same path/plane as you did in the backswing.
If your backswing is correct and on a good path and plane, then simply try to swing the club down that same path on the downswing. Don't try to do anything different or re-route your club by making a loop or any other manipulation. A simple way to have a good downswing path is to try to swing down the same path as you did in the backswing. Too much body movement can throw the path of the downswing off track. So, try and keep your body stable as you swing the club down on the downswing, that will ensure the club stays on a good path and will lead to better golf shots.