This is a great upper-body warm-up. Simply perform a barbell military press and follow it with an overhead shrug, for a total of 15 reps. Keep the arms locked and focus on raising your shoulder girdle up and then down.
You might be surprised at how much you suck at this exercise. I've seen guys that can comfortably shrug 400 pounds and can retract well on cable rows that can't overhead shrug 100 pounds with good form.
To cue the movement, I often suggest a person first perform three reps of regular shrugs just to get the form and to "prime" the movement, and then immediately go into the press and shrug version.
Here we're trying to "wake up" the shoulder girdle. Most males should start with the empty barbell (45 lbs). I've regressed some back to 25 pounds if necessary. If you're rocking it out with good form and good range of motion (initiated from the traps), then go up.
I started with 45 pounds for 15 reps (one military press plus one overhead shrug is one rep) and went up 5-10 pounds a month. Recently I switched to two sets of eight as the weight got a tiny bit heavier.