Agree with the others about trying out different gearing as you ride. Over time you'll start getting a good feel for shifting. I usually try to get as much momentum as possible going downhill ( when on rolling terrain), so will be in a high gear like 3-8' shift into center ring while in base of hill, and downshift as I go back up. I did find that in an effort to keep "spinning" at the same cadence, I was downshifting too fast and losing momentum. It 's a balance act.

For long climbs an easy gear at a steady but effective spin works best for me. But on short punchy climbs, I'm becoming more of a stand and mash type. Trying to work on endurance at standing and pedaling on my commute.

Pinkbikes - great advice. At the mtb clinic I went to last month, instructor mentioned trying not to death grip the bars on climbs. Hadn't occurred to me that part of the why was to not pull up on the handlebars.