Another thing to consider is varying your seated position. Pushed back you will engage muscle groups differently than by being forward, " on the rivet " as it were. On long climbs you can get some relief by varying your position. On long climbs, unless you are in a race, momentum means very little, so don't hesitate to shift to a lower gear and give yourself a momentary break before the climb; settle into a comfortable pace. A large part of climbing is psychological and tiring yourself out early on can make getting to the top more difficult.
One reply wondered where the term " on the rivet " came from, perhaps in jest, but I don't think so. Just look at a Brooks saddle