Many experts recommend picking the gear you think will be good for the climb and then shifting up one harder gear. If you absorb that centripetal force generated by the change in direction upward from horizontal with your elbows and knees, you will loose valuable speed and momentum that could have very well taken you over the top. Do not wait for a significant slowdown before engaging the crank. Keep a higher gear and pedal through the incline. Most importantly, keep pedaling near the top and beyond.
Many people stop near the top because they reach a sense of accomplishment by just getting there.
To be fast in the hills, you have to pedal through the top, not just to it. All you have to do is maintain control as you reach the bottom with maximum speed. Once again, stay relaxed, but do not give into the downward force caused by the changing direction. Keep that momentum transmitting through the arc of the dip so that it will carry you up the other side. If you need to pedal to get up the other side, start early in a higher gear so you won't loose speed.