Deep breathing, 20-rep breathing squats, posture exercises, etc
They worked for many old school lifters. The idea is to expand the ribcage by lengthening the surrounding tissue and allowing for a deeper lung capacity. This in turn increases vitality, endurance, and stability under load.
I've never done specific weight training with this in mind, but I always make sure to breathe deep between sets. I used to do a lot of breathing/ribcage exercises for posture when I had a really bad 'slump' and it worked like a charm.
Form follows function.