#1 - Stick to a color scheme. This is one of the more difficult ideas, but also the one with the most benefit. Having a color scheme in your home or just a room, will give it a coheisive and intentional look. Everything will look like it was put there for a reason. I chose a color scheme of about 4-5 colors for my room. I use to monochrome tones, mostly black, dark grey and white, and then I have details in copper and dusty rose (a blanket, a lamp, etc). It looks clean without feeling to clinical.
#2 - Choose a style. Another extremely important part of having a coheisive look is choosing a style. Everyone's style is different, and of course you should choose one that matches your aesthetic. That could be anything from bohemian to punk rock. My style is a mix of mininal and vintage. I limit the amount of clothes, books and knick-knacks on display to the bare minimum and instead let one-of-a-kind vintage items take up the space on my shelves. Well, not all of them are actually vintage. my record player for instance is from Urban Outfitters, but it still adds atmosphere; just like my Instax pictures on the wall.