Clothing tips for the genetically skinny
V-necks increase shoulder breadth if worn as an outer layer (I have no idea why!)
Horizontal striped clothing (makes you appear wider as opposed to vertical striped that give an impression of height)
Houndstooth and plaid patterns give an illusion of breadth
Generally the smaller your waist looks the broader the shoulders look
Layered clothing (pretty obvious - the more layers the more mass)
Don't make black the outer layer unless you're overweight - black is slimming
Suits are practically designed to make your shoulders wider (I hear this is to do with appearing alpha in the boardroom) if you don't believe it, see any high ranking military uniform
Shoulder seems that actually rest along the shoulders rather than your elbows!
The larger the collar on the shirt, the smaller the shoulders look
Vests narrow shoulders
You may think that baggy is big - baggy is bad unless you are already big
If you're wearing a long sleeve, roll up the sleeves. This gives a 'triangular' illusion i.e. broad at the top and slim at the waist. Why this doesn't seem to be the case for T-shirts I'm not sure...
If you're wearing a T-shirt on its own, skin tight can be good if your shoulders are broad enough to handle it.
Wear jeans that FIT i.e. not baggy. I overlooked this for a long time but even your jeans effect the relative appearance of the upper body.
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And don't forget the number one thing that's going to change your body features more than just superficially: Work out (for actual presence, focus on the deltoids) -high weight low reps. Forget about the biceps for just now unless you shoulders are comparatively broad already - biceps are rarely noticed inside anything other than a T-shirt or something skin tight. The shoulders are going to make the most difference.
Layers is the key! I usually wear 5 to 6 layers especially during winter time, keeps me real warm and can take off any layer when feeling too hot so I make sure it comes with a zipper or buttons. But you have to be really, REALLY careful with the combinations. Use the right colors and definitely not too many (maximum of 3 colours or you'll look like a clown). I've combined an urban style with a latin style, not a quite simple task but I was just going out shopping, bought a couple of stuff not even thinking about combining anything and then when I went home it seemed to match pretty OK. Refined it a little bit and was quite pleased with the results. The only way you're going to find a nice style is to go out shopping and have a good look around. To me it seemed as if my eyes were automatically getting focused on the stuff I liked and then I knew I had to really get just that piece, it was the stuff that made all other clothes in the shop look like shit. If you instantly like something, and can afford it, buy it right away. It will be the piece that is totally congruent with your character because you instantly liked it.