Hold on to a chinup/pullup bar with an overhand grip and hang at arm’s length. Without moving your arms, pull your shoulder blades down and squeeze them together (refer to pic). Men should be able to hold for 10 seconds, women for 5 seconds. If you can’t, you have an upper back weakness and you should make this exercise part of your workout. This movement is key to doing proper pullups/chinups. It will also train your shoulders to be down and back, which will promote good posture.
If you’re unable to perform a chinup check out my CHINUP PROGRAM.