I don't agree with nearly anything RICKY5 suggested there. It's all far too broadly referenced. I can pull things out of my own wardrobe that each exclusively land in the 'don't wear' list, and end up looking very, very good. It's the specifics of an article & you which determine whether it works well or not, from a aesthetic perspective. You can certainly abuse certain clothes, but categories of clothing, like khakis, just can't be ruled out.
My own suggestions would be to:
A) Buy clothes from an outfit perspective. Instead of selecting articles based on their individual merit, choose them according to how they look with a specific outfit, either by buying one whole outfit at a time, or imagining what articles you like at home. Therefore, the design on a shirt, for instance, isn't nearly as important as it's base color or colors, etc.
B) Choose articles that fit you well, and work with your body shape.
For example, I'm relatively slim, so I choose shirts that fit close around my stomach and chest, with well shaped shoulders. This emphasizes the V shaped torso, like what you see male models emphasizing. It's masculine and appealing.
If an article seems pretty great, but doesn't do the right things to your frame, don't buy it.
Due to this, cheap clothes should be avoided. Small brands from small, non-franchise stores are your best bet. Designer is nice, particularly because It's typically made well, fits well, and looks good otherwise, however designer is not a necessity at all. They're just reliably good.
C) Accessorize. Seems girly, but it doesn't have to be. A cool belt buckle, a nice watch or wristband, non-standard eyeglasses, whatever. Just little stuff to break up the potential monotony of an outfit.
D) When putting together your outfit, start with the most significant visible article being worn: 1) Your COAT (if you're going to wear one). After your coat is decided, choose 2) Your PANTS. Color fit and style, etc, that best fits your coat. After those two are decided, the process of elimination narrows things down a great deal, and the other items can be selected to match easily.
E) Due to the aforementioned importance, invest in a really nice coat/jacket or two. If you're gonna splurge on one article, make it your coat. It sticks out the most, speaks the most, tends to be your most 'weighted' article when people get a first impression of you. Choose your coat 80% based on fit, everything else is a side consideration. The silhouette your coat imparts is a huge consideration. Personally, I tend to favor a silhouette that does the most to emphasize shoulders, and does not taper/splay outwards at the bottom. Keep the stomach close, both when zipped/buttoned, and when open. Preserve the V shape.