if you dont have clips or clipless shoes it might be challenging os if you dont then get some would be my first suggestion. i recently did a couple 4 mile climbs with 4 mile downhills (brakeless, yeah i know ******** idea) out in the berkshires of western mass and i think the key is to maintain control from the beginning. dont jsut start bombing the hill then hope to maintain control later on. gradually spin faster until your at the best cadence for the hill. some hills ill go down doing about 25-30 mph but like for a big hill like the ones i did in the berkshires i really had to work hard to keep it down to about 15 mph or less. and of course just keep riding more and as you get more comfortable youll find out the best way for you.