FWIW, best climbing seasons I've ever had were:

- Once early in the season I tried letting a very fit fixie rider pace me up a canyon road until we literally couldn't pedal anymore due to leg muscle exhaustion. The fixie part is important because I had to stay in a much higher gear than I would have otherwise, and stand up and fight just to stay with him on the steep sections. We both collapsed and were near puking, and it took about ~15 mins of rest before we could ride down. Next day, before my legs had a chance to get sore, I did another regular climb. The rest of the season, I was unusually and noticeably faster than my friends. The workout where I pushed to exhaustion gave me a boost I didn't expect.

- Last couple of years I've worked up up to some longer rides, gone out for few super long rides on my own (50-100 miles). Having to work up the endurance to get through multiple big climbs in a ride, I've noticed, really helps me hold my own when I'm back riding shorter climbs with friends.

Generally just riding a lot will help too, technique comes with practice. Good luck!

It's unlikely that a single maximum effort workout increased your fitness much. I suspect that the psychological aspect was more important. You proved to yourself that you could push harder than you believed before. Most endurance athletes lack the mental fortitude to reach their psychological limits.

You could be right, and I understand why you’d jump to that conclusion, but I don’t think that’s what happened, and here’s why. The effect didn’t last all season nor give me any boost the next one. When I slowed down my frequency of rides later in the fall, I felt my performance decline. It made climbing noticeably easier right from the start of a ride, before I felt winded or tired or like I was having to push through anything. Before this happened, I had already done a lot to prove to myself what I can and can’t do, I’ve had other times I’ve pushed myself to the limit. I don’t think this really was a single event, I think it was a boost in the middle of a bunch of rides. It was the back-to-back rides in the context of my lets-get-going early season riding, though I don’t remember exactly how often I was going. At the time I expected this ride was going to make me sore for a while and tired for a few days, like what usually happens after an extra long ride. I was really surprised when it didn’t do that.

BTW, it might be important that the sprint to exhaustion up the canyon was relatively short; it was 5 miles averaging medium steep, with spots up to 12% grade. I usually ride it in granny gear, “speed 1” on a 27-speed bike, smallest chain ring, largest on the sprocket. To follow the fixie I had to stay in speed 14 (middle chain ring, middle of the sprocket.)

This experiment very vaguely reminds me of some methods people use pump up the level of the red blood cells... I might be mixing it up with something, but it's a curious thought.

There's not a huge amount of technique to climbing, it's mostly power to weight ratio. Do you have a power meter and have you done an FTP test?

One technique you can practice is calibrating your effort level in terms of watts or heart rate to the length of the climb. You need to get a feel for the maximum output you can sustain without getting gassed before you reach the summit.

I don’t have a power meter, but for the purpose of pacing I might get away with just the hr? I’ve always been bad with pacing myself, so that looks to be the lowest hanging fruit.

If you have the means and are serious about improving then I highly recommend buying a power meter. I have Favero Assioma pedals which are about the cheapest option for dual sided power and work well. After I've been riding for a while I tend to slack off and fool myself into thinking that I'm working harder than I really am. The power meter keeps me honest.

But without a power meter you can certainly pace yourself by heart rate. Figure out your heart rate zones and then do some test rides holding your heart rate steady in a particular zone and sustain the effort as long as you can. Then you can estimate a maximum target heart rate for each climb segment based on how much time it should take.

You can also experiment with different pedaling cadences. Every cyclist has a different optimal cadence, but in general you want to shift down when climbing and keep the cadence high (unless you're intentionally doing a low cadence strength workout).

Correct me if I'm wrong...

So if I have a power meter I have instant feedback on the power level, and if I know my target power level I can work up to it without overshooting.

HR, unlike power, is a lagging indicator so I am likely to overshoot, especially if I am poor at pacing to begin with.

Yep, HR can tell you a lot (power meters are pricey, but indeed are the best option). If your HR rises too quick you're likely off pace however though.

Another thing you can do is thinking of saving some gas in the tank for the last bit of the climb where you'll sprint as you summit. Start with this in mind at the bottom