The Flaw: Flaring the elbows
The Fix: Similar to a bench press, your elbows shouldn’t point straight out to the sides when pressing overhead; this not only puts undue stress on the small (and vulnerable) rotator cuff muscles but also limits power output. Keep your upper arms at a 30- to 45-degree angle with your chest. Using dumbbells allows you to make this shift much easier than with a barbell.
The Flaw: Always doing them seated
The Fix Seated overhead presses are great and allow you to overload the delts with maximal weight, so keep doing them. But be sure to mix in some standing overhead presses (aka military presses), too, which increase core involvement as your torso is forced to stabilize your position in the absence of the seat. You won’t be able to go as heavy, but the core strength you’ll develop over time will be well worth it from both performance and injury risk standpoints.
The Flaw: Locking out the elbows at the top
The Fix The only thing this will accomplish is a stronger contraction from the triceps. But it also takes tension off the delts, which isn’t good considering the overhead press is a shoulder, not a triceps, exercise. At the top of each rep, stop a degree or two shy of full elbow extension. Save your locking out for arm day.