You improve it by working on it!
Scrap pushups, inverted rows, I's Y's and T's for your external rotators. Face pulls, rows focusing on really getting your elbows behind your back.
If this is a real problem, you should do some stuff in your warm-up, before pressing, to get your rhomboids and lower traps going.
Dumbbell Presses from different angles really help me practice getting my shoulders back and stable.
For the last 2 weeks I've been wrapping flex-bands long-ways around the bench for bench presses. Its really easy to dig in, and get way up on my traps to get tight and stable this way.
Squeezing the bar really hard is supposed to make it easier to lock your shoulders in and maintain tightness. The bottoms up kettlebell press is a good way to practice squeezing the bar tight, keeping your forearm vertical, and your shoulders "packed."
You can do sets of 20-50 band pull-aparts and 10-15 band dislocates, a few times a day, every day, to maintain mobility and practice squeezing your scaps.