The lateral raise is the best exercise for building shoulder width. It isolates the middle delt and creates the broad, athletic upper body that pressing alone will not.
Technique
1Lean forward slightly with a small bend in your elbows
2Raise out to the side until your arms are parallel to the floor — no higher
3Let the pinky side lead slightly at the top, as if pouring a jug
4Do not use momentum — swinging reduces deltoid recruitment
5Very light weights are normal here — 5–15 kg is a lot for most people