"when you stand for a short spurt, drop a gear or two for more resistance, but still aim for a medium cadence"

On long hills, combining sitting with some standing for a break and some air, works for me. As 12strings points out, you'll want a somewhat higher gear when you stand, especially as you get into better shape. In my motion to stand I'm up-shifting 1 or 2 or 3 depending on how I'm feeling. I may only take 20 or 30 strokes before sitting down-while at the same time downshifting back to my easiest gear. As has been said, keep your cadence up. There's a mild hill on a loop I do frequently and I try to make it up to the same point each time while standing; in other words setting a goal and increasing that as you get stronger.