The biggest mistake I see with guys is their clothes just don't fit. The pants are too baggy. The sleeves on their shirts are too long. They buy standard fit shirts that add 20 pounds to their look. I was making ALL those mistakes 7 years ago!
Brand Name is Not Everything
You can see above that I had a nice Polo shirt. It's high quality, but it just didn't fit. I mistook brand names with meaning that they represented me really well. If they don't fit they're not representing you well. I had a closet full of name brand clothes and I don't think much of it actually fit me.
I'm a relatively thin guy and was buying standard fit everything. You can see the bagginess at the midsection added pounds to my look. There's way too much fabric around my arms. The sleeves were too long, so I rolled them up. Nothing is working with this shirt. Sadly, I see a lot of guys with this look everyday.
Now let's look at my pants. Another poor fitting item of clothing that really hurts my overall style. Too baggy. Too pleated. Too cuffed. Bad. You get the picture. Brands shouldn't be the focus. Fit should.
Slim Fit Changes Lives
Slim fit is now a widely available sizing for most brands. If you're a standard sized guy or thin, then you should get slim fit for both your pants and shirts. There are cuts like skinny fit that are even thinner than slim fit. I think you just need to judge what size works for you. The key is eliminating unnecessary bagginess. It's a style killer.
A simple change of fit can make a major difference. My build is pretty much the same as it was 7 years ago and below is what slim fit clothing does to my look.
How to Shop
Find a brand that you like and then find the fit that works for your physique. Be very critical of how things look on you. Many guys think slim fit is only for super fashion forward guys. It's not. It's available at all major retailers and it represents the new norm. You should push to look your absolute best. Well fitting clothes are critical to doing that, so own it and get clothing that makes you look awesome.
I saw the light about 6 years ago. I switched to slim fit everything and it's made a huge difference. I get compliments on my clothing. Never really got those before. I also know I look my best. With the right fitting clothes you'll look better, feel better about yourself and be more confident. Compare my look from 7 years ago to the one below from The Grammy's with the right fitting outfit. BIG upgrade.