Great question — answering as the UphillTech team:
My hack without any sewing. This wouldn't work for a shirt, but worth trying with a jacket. Lateral thinking . . . instead of making the jacket narrower, make your shoulders wider to fit the jacket. Indirectly, by fitting shoulder pads inside the jacket. Buy some shoulder pads, and fix them inside the jacket's shoulders with a latex adhesive such as Copydex (no affiliation). The advantage of this brand is that it remains flexible, and the pads can be removed by gently peeling. If there is any remaining adhesive just press a small area of cotton fabric onto it. One disadvantage is the initial slight fishy smell, which soon goes.
Full answer by Weather Vane: https://lifehacks.stackexchange.com/a/27640 (CC BY-SA 4.0).
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