Spinning takes practice. Eventually, you can go faster and have more in reserve for bombing down the descent if you pick a gear that allows you to maintain as close to your flats cadence as possible. You may start out slower at first. When you get stronger, you can spin as fast in slightly harder gears, thus upping your speed. Or, conversely, you may be able to add another 10rpms to the original easy gear choice. I'm not a strong climber at all, but by taking the time to practice this technique, I have been steadily improving. It's really hard to at first, because you think, why do I want to go with an easier gear if it's just going to slow me down? Over time, it won't. You may even have a little left over for a surge at the top! The really steep stuff where you run out of gears where you can spin will keep your strength up. And you can power up the short rollers. I find that if I don't want those climbs a mile or longer to kill me, I've got to spin. I may fall back at the base behind my huge gear using riding buddy, but I almost always pass her at some point later on in the climb.