Hold dumbbells in both hands. Keep your elbows slightly bent, slowly raise your arms outward until they're parallel to the floor or slightly higher (don't force them too high). As you lift, slightly tilt the thumb sides of your hands downward (as if you're pouring out a little water for a bottle). This better isolates the mid-and rear deltoids and rotator cuffs (the front deltoids get plenty of work with other upper-body exercises; in fact they're often overworked). Pause, then slowly lower the weights to the starting point. Don't tighten your neck, pulling it back during the lift, which causes neck strain.
Modification: If you have shoulder or neck problems, do this exercise on a machine or with tubing or free weights one arm at a time while supporting yourself. Do not lift as high, and stay just below 90 degrees, keeping your hand below the level of your shoulder until this gets more comfortable. Go higher gradually as tolerated.