I live in Colorado. I thought the key to my first century was to ride a lot of miles - so I rode a lot of flat miles. Although I finished, it was just barely! The key to my second century was to climb, climb, climb! I can't tell you how much better that turned out to be!
As for gearing, check out this link on gearing for the Tour of the California Alps (Markleeville Death Ride).
http://www.arniebakercycling.com/pub...ll%20Gears.pdf
Good luck on your next century! You're almost there! Embrace the hills!