Before going up a hill, try and prepare for the hill in advance by being in the right gear. Also, on a particular steep part of my ascent I push hard on one pedal, then do two pretty easy revolutions, then push hard on the other pedal, and repeat. I used to get so fatigued spinning constantly now I don't get nearly as tired. I forgot where I got that advice from or if those are the right terms, but it worked for me.