Something I underestimated is shifting skills and momentum. This works for rollers, mostly. I spent a month on Vancouver Island this summer and it was ALL rolling hills, everywhere, and pretty serious hills in many cases. I have improved my shifting a lot, to a point where I can now sometimes pass my Dear Partner - a MUCH stronger cyclist than me, especially on hills - on an uphill with no hopes for him to catch up. I'm pretty sure that fitness is not the factor here, but strategy.
I thought he was just being nice in letting me pass him and not re-passing me immediately! But when he told me he actually couldn't catch up I started observing what I was doing on those specific occasions. I realized that, on the end of a given downhill, I'd shift up (or increase cadence if no bigger gears were available) and take all the speed I could find. Then I would start the climb and as soon as I started feeling resistance I'd shift down pretty quickly, maintaining a very high cadence.
In other words, it means no real resting on the downhill. But a really fun time on the uphill!!!!
In any case, practice is the key. There is no other way to get good on hills. Sometimes slow, sometimes fast, in different gears, trying different tricks, sitting, standing, singing (it helps). Smiling at the hill is also a booster! I think it makes the hill shy away.