I agree with those who say that you have to "think" your way over the hill, to have a strategy for a hill that takes into consideration the length and grade of the hill, the energy that you have available at that time, and your underanding of how much energy you must devote to this one task without over-expending yourself.
Two things are frequently overlooked in this area:
1-rapid downgearing at the bottom of a hill is psycologically defeating. To quickly jump to the lowest gear, and then to discover that that is it -and yet the effort is still uncomfortable, is a negative way to approach the challenge. If your in with a group and that grating sound is happening in unison only compounds this effect more. Bikes generally find it easier to upshift rather than downshift on the cassette since the chain is being thown to the small cog-especially under stress.
With experience it should be possible to approach the hill both positively and agressively, using a combination of climbing techniques such as sliding back on the saddle, standing and upshifting and generally being less gear-reliant. Focusing on the top of the hill, rather than keeping your head low and seeing it as an ordeal will help. In other words you are throwing yourself into the hill, rather than trying to level the hill by maintaining flat road cadence and having the gears attempt to negate the hills. That in a way is doing a disservice to the hill and there should be no reward for it. Why do hills unless they can add in some way to the overall experience of riding?
2-Too often we envision the effort of climbing to be a joint effort of the lungs and legs. We forget the advantage of leverage. This advantage varies according to where you place your hands. The upper body has a lot to do with how you will climb effectively, and the implementation of this advantage changes considerably between hands on the drops, hoods, and especially in the standing position. There is an article somewhere around here that relates to this. The writer uses the example of putting a bathroom scale next to a wall and registering the force that one can apply with one foot on the scale. That is compared to having something attached to the wall, 3 feet above the ground to simulate handlebars, and bending over and holding the bar substitute and then pressing on the scale with full force. The result will show the advantage of leverage and the utilization of upper body strengh in the transference of power to the pedals. If we understand this then we know that we are being carried up the hill by more than just our legs, and that strategy in the way of a combination of tecniques will assist in accomplishing the immediate goal.