I’d suggest picking a theme before you start with some of the other suggestions. Not necessarily a theme like French colonial (although feel free, if you’re into it) but a particular style you have/like (modern, clean lines vs. traditional, dark wood, for example) or a color scheme or a real “theme” like decorators talk about, like Old World Pub or Sports Fanatic*.
Also, ask yourself what you like about the well-decorated places you’ve seen, that will also give you some ideas.
Then whatever pictures/furniture/colors should compliment that. It’s better to have a fairly good idea of how you want your room to look before you just buy crap that you like and throw it all in there regardless. I think that’s the real difference between a well-decorated room and something just blah, or even atrocious.
Here’s an example of what I did for very little money:
My living room is a retro sailor-themed room. I know, you’re thinking, what the hell is wrong with you? It actually looks, IMHO, pretty cool. I chose red and black for the bold colors in the room (the walls are neutral, wood floors, white baseboards and trim). My couch is a neutral tan with red throw pillows with a red ottoman in front of it. I have an old black leather chair that’s very retro (it is an old office waiting room chair circa 1960). I have a simple bookshelf painted red and black. My TV stand is an old cedar chest circa 1960 as well. The end table, couch, chair, chest, and desk, and dining room table are all either vintage or vintage-inspired (the couch’s description said something about based on the design of furniture in a 1930s hotel). I got it all for free (liberated from my parent’s over-cluttered house, and dumpster diving) or cheap (garage sales) except for the couch and ottoman. For art, I have a Rosie the Riveter poster (not strictly sailor, but worked well), a vintage Navy recruitment poster, an anchor, and a giant tall ship sign my friend bought from a bum for $4 and a cigarette to apologize for an argument we’d have. If I ever get around to it, I’m painting it to fit the rest of the room. I also have a lamp with a ship’s wheel on it I painted red and black.
I didn’t go overboard on the sailor theme. If you remove the wall decorations and the lamp, it looks like a funky hotel lobby from the 1930s (some eras work better together than others- 1930s and 1960s, at least with the furniture I have, is hard to tell apart). Total cost of everything in the room, including furniture, paint, and decorative items, less than $500 spent over the course of a couple years.
My kitchen is done in a similar style (I live in an Art Deco building, so the architechture fits a 1930s style). I started off with the colors baby blue and yellow. I bought a retro yellow step-stool and a baby blue collander (used as a fruit bowl unless I make spaghetti) then yellow and blue towels, and I had an old vintage French food calendar. Once the calendar was useless, I took the pics from it and framed them in yellow and blue frames.
I like funky, retro stuff. It’s not for everyone, but it fits my style and works well in my apartment. Most of my furniture/stuff has been amassed over several years, but because I like that particular style, it all works fairly well together.
Just take a look at your current furniture- is it mostly dark wood and traditional? Light wood? Black? Do you lean towards traditional, or more contemporary? That should give you a good idea of what you have to work with, and you can go from there.
Then, like other posters have said, brighten the place up with some paint (if possible), bright pillows, pictures and posters, candles. Contrasting colors really make a room pop. If you have a lot of blue and white, a few spots of orange (whether pillows, flowers, or candles) will compliment the blue. The same goes for yellow and purple, red and green. Or you can pick a cool pallette (blues, greens, purples) or a warm pallette (reds, yellows, oranges). A room full of neutrals will also look great with one or two sparingly used accent colors.
Go dumpster diving (actually climbing into the dumpster is not actually necessary… though not as gross as you think if it’s behind a used furniture store or something). If someone has a pile of furniture next to their trash, take a look. Some of it might actually be in great shape, the person is just downsizing. Garage sales and thrift stores are also great. Don’t forget you can paint or stain whatever you buy or find, so a dirty scratched surface can look great with $20 and a couple hours of work.
I second the fruit bowl idea, although I’d suggest real fruit if you will actually eat it, it looks nice and is good for a snack.
*Try not to go overboard with a theme, it looks cheesy.