For me, downhill putts are scary putts. Ot at least they were until I learned a few tricks to put the brakes on those little devils.
This is what I do and maybe it will help you as well.
Stand next to your ball and make several practice strokes as you normally do. Keep stroking until your mind's eye sees the perfect length and rhythm for your stroke. This is something that you will 'feel' as much as you will 'know'.
Once this perfect stroke has registered in your subconscious, immediately set up over the ball and lower your hands (choke up) at least three inches (and sometimes as much as six) down the grip. Now this will feel odd the first time you try it, so don't let that throw you off.
Keep in mind that the shorter the putter, the less energy will be transmitted to the ball. In other words, by choking down on the putter you are delivering less energy to the ball and it will therefore roll softer. And that is the key to making these downhill putts. Soft...soft...soft.
Now make the exact same stroke you just practiced. You'll be surprised how softly your putt rolls while staying on its intended line.
Using the 'short putter' method works better (in my opinion) than a toe-stroke to soften the blow, because the ball is struck on the putter's sweet spot. The putter face remains square and putts start on-line (where as toe-stroke putts tend to start to the right). Having your ball start on-line is another key to making these putts.
The faster the putt, the farther down the grip you go. If the putt is ridiculously fast and ridiculously fast putts do exist go all the way on to the bare metal shaft with your grip. Again, this will feel odd at first but give it a go and see if it works for you.
When you learn how much break to play with this technique, you'll start making a few of your downhiller putts.
As with everything in golf, practice is important. This is really not the kind of technique that you want to try first time while playing with a group. Much better to find a hilly practice green and spend 30 minutes or so alone. Knowing in advance that you know how to play these lays will greatly increase your confidence the next time you're faced with one on the course.