1) Frequent power cycling. CFLs will last much longer if they get lit up and left alone than if they are turned on and off every few hours. The older ones were worse in this regard than the newer ones.
2) Wrong installation orientation. In the tiny type on the packaging, CFL's often say "to be installed base down" or base up. Lifespan will be GREATLY reduced if you put a base-down bulb in a ceiling fixture, or even a sideways fixture.
3) Timer-triggered fixtures. CFL's require a jolt of juice to fire up the ballast and create the spark that then sustains the glow. Many timers provide a current ramp that doesn't quite meet this demand (the power gets there eventually, but not all at the start). Many CFL's say on the packaging, "not to be used with automatic timers" for this reason. Those that are timer-OK cost more, natch.