CFL bulbs are inductive loads and they work best with nice smooth constant voltage. Make sure your motion sensor or dusk to dawn switch is compatible with CFL lighting. Some of them send a flickering on/off cycle to the fixture every startup or voltage leak which doesn't bother an incandescent much but it will kill a CFL quick. A fully enclosed fixture may also cause the ballast to overheat.
I use a dusk timer light switch with a built-in relay. It comes on at dusk for a pre-determined # of hrs each night. The solid "on" relay is compatible with all lighting fixtures up to 500w.
http://www.electrical101.com/cfl.html