The more hills you ride the easier it gets. You get in better shape and you learn how to attack the hill. When I first started riding the best advice I got was to go slow and it's okay to mix sitting and spinning with standing. Sitting is more efficient, but when you stand you can push a harder gear. So by alternating between the two you are able to take advantage of your strength, cardiovascular, and different muscle groups. But the bottom line is climbing gets easier the more you do it.