Remember the top means the top, not near the top. Crest the hill, then prepare for your descent, you will have time to catch your breath on the downslope. Test your brake or adjust your speed at the start of the downhill, not before it. If your are racing for time, downhills are an easy way to gain time with little effort, and gets you rolling fast enough to kick it up the next incline. The same principal applies if you aren't skating for time, so it is not necessary to descend at a rate greater than your ability to skate.
Downhills make the transition to uphills easier.
If you are comfortable striding at 20 mph, going 35 is fun, but not necessary if you have to slow down to the point where you can begin skating again. Stay low to the ground, a lower center of gravity allows gravity to pull you down the hill, and reduces wind resistance. This stance is very stable and aerodynamic, but takes muscle control, so practice, practice, practice. Standing out of the crouch allows wind resistance to slow your speed, and T-stopping is the preferred method of the sidZone, but slowing your descent at the top with a heel brake is important, especially until you gain confidence in your downhill ability. Rest your body, not your mind.