When facing a severe downhill putt on a moderate to fast green, distance control can not only be difficult but nerve wracking as well.
One of the biggest problems comes when players try to be too gentleand hesitant in making the putting stroke. Obviously one must be aware of the fast conditions when putting downhill, however a golfer who does not make a confident putting stroke will most often miss these slick putts.
A good way to convince yourself to make a smooth and firm stroke is to line the ball up toward the toe of the putter. By setting the ball up in this position you will take some off the "jump" off the putt. Because the ball will be struck off-center on the putter face, some of the "jump" or "pop" that is normally produced when striking the ball on the sweet spot is taken away. This allows for a stronger, more confident stroke, without risk of blasting the ball ten feet (or more) past the hole!
My experience has always been to get very comfortable on the practice green prior to starting a round of golf. I don't practice the 30 footers on the practice green as many people do. I practice the 3-6 footers. In getting really comfortable with these knee knocker putts (3-6 feet) I am confident enough to take a good solid stroke at the ball because (even downhill) because I have the confidence to make the putt coming back up the hill should I miss the first one.