Shift gears often. Luckily our bicycles are equipped with a lot of gear options to help us keep spinning comfortably. However, sometimes on a long ride, like a century, it is easy to forget about the mechanical advantage that our equipment can provide.
On gradual or short climbs I sometimes forget to shift and start muscling up the ascent with a slow grind. While I can do this on shorter rides, it increases the possibility of muscle fatigue on a century.
So when my cadence slows, I shift into an easier gear, shifting in the front chain ring for big resistance change and shifting on the rear cluster for fine-tuning my cadence.