Riding uphill is a great way to increase muscular endurance, which is the ability to pedal a relatively large gear at a moderate cadence for an extended period of time. It's effective because cyclists tend to reduce cadence and increase average effective pedal force when riding uphill (i.e., push harder on the pedals). One way to significantly boost your power output is to progressively overload climbing distance.
For instance, start by doing hilly rides with 1,000 feet of climbing and slowly add distance until you can complete 3,000 feet of climbing in a single outing. Another way to enhance power is to do short, high-intensity sprint intervals up steep hills. These sprints should last for 60-90 seconds. Simply ride downhill to recover and then sprint up again. Build up to 12 sprint intervals in a single workout.