Buoyancy by and far is number one. When you have a great understanding of buoyancy, you'll make better decisions on how you're weighted, you'll be able to trim easier, it'll make yo more relaxed - all of that leads to better air consumption.
And this - That is an understatement of the century. When your buoyancy is on, you'll be able to just stop moving - any movement is deliberate - when you have great diving control, everything is in super slow motion. Patience, a slight fin kick and glide is way better than rapid kicks, sometimes you over react and have to counter that motion - stop the wasted energy.
i try to tell my students to try and slooooowwwwww down everything they do. then when they think they have slowed enough, slow down even more.
And this - That is an understatement of the century. When your buoyancy is on, you'll be able to just stop moving - any movement is deliberate - when you have great diving control, everything is in super slow motion. Patience, a slight fin kick and glide is way better than rapid kicks, sometimes you over react and have to counter that motion - stop the wasted energy.