I'm wondering the same thing. I'm a serious hill slug and would LOVE to increase my power on hills. I read somewhere that doing leg presses, squats, and hamstring curls would be good (but I have a bad knee, so squats are out). I think anything you can do to strengthen the quads/hammys will help with strength output on the bike.
I'm looking forward to other great tips from these ladies here who have so much to offer!