Good posture. The upper body is the same while sitting or standing. The skeleton can hold the body's weight. It evolved to hold itself up without using the muscles.Relaxed.Lead with the abs. Abs move first to suck in air. Abs move first to push out air. Focus on the middle of the abs, up and in, in the middle/center in line with the bottom of the rib "wings" on the sides. I think that's also some power chakra spot.You can purse your lips or breath through something like a straw or mouthpiece for extra resistance, like lifting a weight with your breathing muscles.I think that's about it. Crunches or situps can help a bit to tighten and strengthen everything up a bit.
Someone into reed instruments tell me if this is true:An oboist i knew years ago said he could fool the alcohol baloon test with his breathing technique.
That would be amusing if I were having a heart attack and was sitting around observing it while I'm dying.
Wouldn't be a bad way to go though, to die from practicing.