I monitor (without gadgets) three things. My RPMs, my lungs and my legs. My goal at the very beginning is to keep the speed up until the legs start (only start) to feel a bit anorobic - then I start shifting down to maintain my RPM (90+), I will cheerfully keep shifting down as far as it takes, I watch my breathing to make sure my lungs are keeping up with the demands of my legs - if not, I shift down again. Then, as I start hitting the crest, and my breathing slows, I start shifting up again.

The goal - my legs are never blown, and my breathing stays pretty constent. I don't rest on the crest at all - but just keep going. I sort of have to since all Costa Rica is around us is one hill after another.

Making sure that I am spinning with clipless pedals is a big help as well, in fact, sometimes, near the top, I will start focusing on pulling up more - at times, it is all I really need.