Dumbbell pull overs. BUT NOT THE BODYBUILDER’s TYPE!
These are designed to give you arch, not stimulate your chest/lats.
Lie with your upper back on a horizontal bench, and the rest of your body in the air, your feet firmly planted on the ground. Suck your tummy in, then drop your hips as low as your flexibility allows without moving your rib cage. Once you are in the arched position keeping your abs braced, flex your entire upper/middle/lower back, and try to puff your chest out. Now you should feel like you are incredibly tight, and about to explode. Then do some light, breathing dumbbell pullovers. Stick with a 8-20 rep range.
Once you are pumped, practice puffing your chest out and getting that same tight arch whilst standing up, and also whilst lying on a bench press. If you are tight enough, you should struggle to unrack the bench press because you are so ‘in position’.