Han has summed up the basic points really well

When you approach a hill, no matter your ability, it is so important to get into a rythym you can maintain right the way to the top.

Han is so right... if you charge at a hill, you will blow your heart rate and the lactic acid will start to burn in your legs and you will slow down and struggle as you near the top. But if you get in your rythym and ignore the speedies who charge the hill, it is likely you will get to the top before them. If you don't, you will prolly catch them on the downhill and be ready to attack on the flat, or be fresher for the next hill than they are.

When you keep your HR or the lactate under control, you are ready for the next demand. Often the people who charge up hills faster than they shouls for their level/ability will spend 5-10 minutes once over the crest trying to recover and cycling with 'dead' legs while they wait for the sting/bounce to come back into their pedaling.