Let's face it - most men, especially single ones, don't really know where to start when it comes to decorating. Usually we rely on our girlfriends or mothers to direct that sort of thing, or we just buy what we like and hope it all goes together; and then wonder why our apartments look nothing like the showrooms at IKEA. So we're going to help you out here, bro. Cohesive interior design is really just about being consistent, adding some personal details, and buying good basics - much like building a good wardrobe. If you buy a good couch, and a good bed, you can make lots of other design decisions cheaply that will transform your space.
1. Clean Lines: If you pick some simple basics with clean, modern lines, you can't go wrong. Just make sure that you decide what style you like, and then stick with that - really good interior designers can mix modern and traditional pieces and make it work, but you're not an interior designer, so stay on the safe side. Don't choose a puffy, bloated reclining sofa just because it's comfortable, either - pick a clean simple couch in a neutral color, and make sure it's good quality. Same with your bed.
2. Quality Basics: we touched on this above, but you should invest in some good-quality basic pieces when you decorate your place. These basics include your bed, your sofa, and your dining table and chairs. These are the items you're going to use the most, for the longest time, and they're the staples of your design aesthetic. You can change the artwork or the rugs or the pillows, but you can't change the look of your larger pieces, so choose well-made, versatile furniture for these basics.
3. Keep it Clean: the quickest way to ruin the look of your place is with clutter. We all have a bunch of shit that ends up in little piles - the key is to keep them out of sight, or in a designated place. Put your dirty clothes in a hamper, not on the floor...put all your mail in a basket under the counter, hang up your towels. You've made the effort to buy some nice things, don't cover them with coats or magazines. And change your sheets and make the bed - women notice that.
4. Personal Touches: the key to making a space your own is that it needs to reflect your taste. You can buy every piece of furniture, lamp, and rug in a design showroom but that doesn't personally reflect who you are. So pick some accessories for your rooms that you like - a cool vintage poster; a stack of your favorite books on display; an abstract painting you made in college; a pair of cool industrial-looking lamps; a series of black & white photos you took on a trip to Yosemite. It is your space; personalize it with the things that mean something to you. Unless it's your childhood teddy bear. Keep that shit under your bed.