Bad form on shoulder day is a fairly common occurrence. If you're lucky, you'll find yourself the object of a gym-fail video on YouTube. If you're unlucky, you'll find yourself with a rotator-cuff injury.
We all make mistakes, but we can and should learn from them, I've identified nine of the most common blunders I see people commit on shoulder day, and provided ways for you to correct them.
1

Never assume all overhead presses are equal

The overhead press may be your go-to multijoint movement for shoulders, but that doesn't mean all its variations work your deltoid musculature the same. Not even close! That's why you should understand how each variation works a bit differently so you can pick the right tool for the job.
For example, lowering a barbell in front of or behind your head actually changes the emphasis on your deltoids, as does your elbow position relative to your torso. When you press overhead with dumbbells, your upper arms go straight out to your sides; that's your cue that your middle delt is positioned to undergo maximal contraction.
When you do Arnold presses or barbell presses in which the bar is in front of your head, notice how your elbows pull forward—they're no longer directly out to your sides. That slight shift recruits the front delts and relieves some of the tension from the middle delts. While that doesn't discount the effectiveness of these kinds of presses as great shoulder movements, they're not the best at emphasizing the middle delts.
Arnold press

Dumbbell and barbell presses in which you take the bar behind your head will better target the middle delt region. Machine overhead presses differ by manufacturer, but if you keep an eye on the direction of your upper arm, you can easily pick out whether you're getting much contribution from the front delt.
If you want more tension on your front delts when doing overhead presses, by all means choose the front barbell version, or consider Arnold presses.
2

Never overarch your back doing overhead presses

All kinds of things can go wrong with too much weight on the bar, especially when you press that weight over your head. In the all-too-common scenario where you bend backwards to press more weight, your body searches for a way to gain better leverage. This can quickly turn an overhead press into a steep incline press, which recruits the chest musculature to a greater degree puts your lower back at risk.
Note that this overexaggeration of the spine comes with a steep price, and I'm not just talking about the reduced stimulus for your delts. To press safely, you want your spine in a neutral or slightly arched position with your core muscles—both on the front (abs) and back (low back)—braced and contracted. But that's not what's going on here. The hyperextended position of the thoracic and lumbar vertebrae combined with heavy weight can contribute to a herniated disc.
When doing seated overhead shoulder presses, keep your back pressed fully against the bench. When doing them standing, be aware of overaggressive arching. You'll certainly want a degree of arch in your back, but don't turn the exercise into a dangerous standing incline.
When doing seated overhead shoulder presses, keep your back pressed fully against the bench.
3

Never go super heavy with behind-neck barbell presses

While some lifters slip into a 3-sets-of-10 mentality for the bulk of their training career, many others cycle heavy and lighter training periods. While doing low-rep training with heavy weights can help build stronger delts, it might be a mistake when you're loading extra plates on the bar and repping it behind your head.
When doing a behind-the-neck barbell presses, the shoulder muscles are in their weakest anatomical position at the bottom of the move. Using too much weight on this movement can increase the risk of a tear, according to Guillermo Escalante, DSc, CSCS.
Escalante recommends going in front of the head if you're going to use very heavy weight, although doing the behind-neck overhead presses with moderate weight is a safe way to target the meat of your delts.
Behind-neck barbell press

4

Never use a close grip on upright rows

To work your middle delts maximally, you want your upper arms moving directly out to your sides. That happens by taking a moderate grip on a bar when doing upright rows, not an extremely close grip.
If you take a close grip on the bar, watch where your elbows go. They move forward and out to your sides, not directly out. Using a close grip causes internal shoulder rotation, which isn't always healthy for your joints. This not only put less emphasis on the middle delts, but also increases the likelihood that you could have long-term shoulder damage.
Push your grip wider on the bar to allow your elbows to kick directly out to your sides, which is perfect for building up your middle delts.
Push your grip wider on the bar to allow your elbows to kick directly out to your sides, which is perfect for building up your middle delts.
5

Never limit your range of motion

Many trainers terminate single-joint movements like lateral raises and front raises when they hit roughly shoulder height, but the middle and anterior delts haven't reached the ends of their range of motion. In fact, you can take these movements even higher, as the muscles are still contracting. Some experts suggest you can go as far as 45 degrees past the horizontal plane.
The longer range of motion makes these movements harder, so you may sacrifice a bit of weight, but the extra range and time under tension makes these moves worth adding to your workout every so often in conjunction with heavier single-joint exercises taken to about shoulder height.
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As Of 8/18/2015