A few more ideas:
Have some sort of a plan for a look you want. It doesn’t even have to be specific, just take some time and think about styles and looks you like. If you keep that in mind, it will be easier to pick things up when you see them, whether it’s in thrift stores or department stores.
Sometimes nothing is, in fact, better then something. If you have a sense of what you like, and what you hope your apartment will look like eventually, try to gently decline things that really don’t fit in. For example, when Great Aunt Martha offers you curtains that are not a good match for you and your taste, I would politely decline rather than take them. It’s tempting to think “oh, I’ll use them until I get something better,” but this often leads down the path of having them forever. You don’t want to have stuff that you don’t even like.
If you can paint, paint. Painting one wall is a heck of a lot easier than painting a whole room, if for no other reason than that you can shove all your stuff up against the opposite wall while you are painting. You can also think about a bolder, more dramatic color on just one wall, if you’re worried about the color overpowering the room.
I think the days of having one or two colors for the entire room are behind us, but it’s still a good idea to pick one basic color to use as your foundation. You will always be able to find things of other colors that will complement your main color. Working backwards, if you already have a couch, its color might be your room’s color by default. It’s also easy to remember one color, so when you go out to purchase things like coasters, or napkins, or whatever, it will be obvious what color you’re looking for.
If you hate your couch, slipcovers are good, but in my experience, it’s not that easy to make a slipcover if you are not particularly crafty to begin with and not have it look like … well, like a homemade slipcover. You might be better off hiding the bulk of the couch with a comfy throw that you do like. Solid, if your couch has a print. You don’t want them to battle to the death or anything.
There are all sorts of things you can hang on your walls that are not art, or at least, not art in the traditional sense. I am not a huge fan of the framed museum poster because it seems sad to me to have something that thousands of other people also have on their walls. However, I do think framed posters work if you can get some sort of theme going on – maybe if the subjects have some special significance to you, or to the area where you live. Other things you can hang on your walls are empty frames, mirrors, windows (the old kind, in wooden sashes), and stuff of any sort, as long as it is mostly flat. If the stuff in question is heavy, it’s worth it to talk to someone who knows how to attach it to the wall properly, you don’t want to damage anything.
I like big things on the walls, but several smaller frames in a group can also be neat, especially if you play around with the way you arrange them. You can frame snapshots of friends, and rotate them every once in a while so you don’t get bored. You can matte them yourself, and use a color other than white to spice up the room.
Oh, rotating stuff is one of my favorite things. If you collect something, rather than display your entire collection, display just a few things at any given time, and change them frequently. It’s easier to keep clean, for starters. Plus, you look less like a crazy person.
(I say that as a person whose collections tend to run away with her.)
Another thing I like is a nice vase. If you are on top of things, you can put actual flowers in it, which is traditional and always nice. However, I am not that on top of things, so I’ve found some interesting things to put in vases over the years. Currently, I have a bunch of old art paintbrushes in a vase, they look well used and interesting, in that you don’t see them in vases very often. I also had a vase filled with old arrows from my archery days (pointy end down, we don’t want anyone to lose an eye) – so any hobbies or interests you have could also double as decor.