With any exercise, to continue to receive the full benefits of the exercise, one has to continually increase the difficulty to avoid a plateau.
A general rule of thumb for increasing resistance is to do it once you are able to complete the upper number of reps within your range with proper technique. For instance, if your range is 8-12 reps per set, you should be able to complete 12 reps with proper technique and ease. Once this is achieved, you should consider increasing the resistance by roughly 5%. Keep in mind, however, that increasing your resistance will affect the number of reps you can perform until you grow stronger.