The Bodyweight Bridge
Don't worry, no one's trying to get you to train for the circus. This movement helps improve overhead shoulder range, T-spine extension, and helps open up the ribcage and hips. These are huge players for big muscular lifters.
As you can see in the video, the idea is to get the shoulders to stack properly above the arms. Note how I attempt to work my way into "drifting" forward over the shoulders to help groove myself towards ideal positioning. This is something that a 155-pound lifter can easily master, but is that much more impressive when a 250-pound lifter can pull it off.
Focusing on sets of 10-15 seconds can go a long way in boosting your mobility and prepping your entire posterior chain for your workout. Alternately, you can do them on their own during a dedicated day of bodyweight exercises.
Mobility and flexibility work may seem redundant, but if it's extremely difficult for you, it's probably indicative of a link in your chain that needs attention. Bodybuilders and meatheads take note.