Skinnier men can easily add stripes to their wardrobe. Choose thin stripes and patterns that are close together, either vertical or horizontal. This adds the appearance of bulk, especially if you use horizontal stripes or patterns. Make sure the shirt fits nicely; you want the fabric to cling to your form.
Skinnier men also have the option of choosing any color that appeals to them. Where larger men need to stick to dark colors, skinny men can mix and match. Adding contrast breaks up an outfit's lines and can add the appearance of weight. Light jackets paired with a darker shirt can add the guise of bulk around the chest and waist.
Do not wear sleeveless shirts. Tank tops and sleeveless t-shirts will highlight the fact that you have little to no arm muscle. Stick to well-fitted t-shirts, dress shirts, and long-sleeved t-shirts.
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Fabrics with sheen can add the appearance of weight. Silks, rayons, and satins can all make you look bulkier than you actually are. Consider adding a couple of shiny shirts to your wardrobe. I am not talking disco shiny, but something with sheen.
Being skinny does not mean you must dress like a stereotypical nerd. Choose fashions that you like and that do not drown your body shape. Follow these simple rules and you will look brilliant in no time!