You might want to look at Sheldon Brown's page on shifting as a starting point: http://www.sheldonbrown.com/gears.html

You should be pedalling at a steady cadence all (most!) of the time. Shifting gears allows you to maintain that cadence even while things like road and wind conditions change. For example, when the road heads up and makes is harder to pedal, shifting to a lower gear then makes it easier again.

What are you dreaming about in the realm of long distance bike tours?

--- Denise