centrifugal: Moving away from a center of rotation.
centripetal: Moving toward a center of rotation.
Remember that a centrifuge is a machine that separates liquid compounds, such as blood, by spinning them. The heavier elements of the compound move away from the center, separating from the lighter elements (in the case of blood, it separates into hemoglobin and plasma). Most carnival rides that involve spinning rely on centrifugal force to keep you from falling off the ride.
I cannot think of a single thing that uses centripetal force. According to Webster's New World Dictionary, centripetal is used in botany to describe certain flower clusters that develop inward instead of outward. Perhaps that is how you can remember the difference, by linking flower petals with centripetal.
Can you think of a common example of centripetal force? Meanwhile, I'm going to enjoy the rush of centrifugal force on the teacup ride.