Assuming the bike fit is good I’d say the other thing to concentrate on is keeping your legs moving as much as you can whenever you can, it’s a conditioning thing IMO,
Basically you get your body used to pedalling by doing more of it… for some reason I find coasting along letting the freewheel click away has a noticeable effect on my legs/pedalling, even if your not “Under load” I think it’s best to keep the pedals turning as much as possible, stops you legs cooling down/seizing up, the last thing you want is to arrive at the bottom of a climb in that sort of condition…
I don’t know if you are exclusively an MTBist, or dabble in the dark-side OP, but I honestly believe commuting on the road by fixie has helped me generally with pedalling technique and MTFU-ness, I’m still not what I’d consider “fast” but I can keep the pedals turning now whatever, you just get used to it because you have to… one thing the fixie highlighted for me was just how often I stopped pedalling and let the FW do its thing, so I make an effort to do the opposite now…
You don’t have to take up comedy death bikes, you can just make the conscious decision to spin more whenever riding, it honestly helps, Comfort on the SPDs is a bonus, if your feet are comfortable, and your knees straight, you’ll make better circles…
The other thing too is that I don’t like stopping if I can help it now either, I used to be fine with a bit of trail side chatter but not so much any more.
I’d say it takes me 3-4 miles at the start of a ride just to get warmed up and comfortable, stop for just a few minutes and it’ll take me another couple of miles to get back into the swing of things, I don’t think I was always like this, might be related to age or changes lifestyle/sleep/stress/kids/etc, and I’m not sure others I ride with have the same issue or require a bit of “Restart time” all I know is I prefer not to stop, to avoid being pointlessly bolloxed…
All the other standard things apply too; Diet, Sleep, Rest, etc…