Number 1: A headboard. Yep. It's just as simple as putting up a headboard. I found that as soon as I put mine up, that my room seemed to represent my style... Which I feel is a little more modern-istic.
Number 2: A rug. Whether you have hardwood floors or any other type of flooring a carpet is going to help define the space. Seriously... I found this little baby at the Salvation Army for $45, and it's possibly the best addition to my bedroom.
Number 3: Curtains. Frame those windows!! Seriously, though, do it. It not only keeps the creepers away, it can add a lot of character to the room, depending on what kind of style you pick. I used to have these little old grandma ones that were yellow and white stripes... YIKES! Now, though, I have these gorgeous white ones. Simple and very easy to take care of. I like them because they still let some light in during the day.
Number 4: Bedding. Okay, so we all know that I adore my pin-tucked duvet cover. It's really marvelous and I love it. On top of that, though, I add pillows and printed sheets. I like to mix the kinds of pillows I use, and I admit I have some super nice ones that I've never slept with. I have to sleep on the pillows that are just barely holding together... or else I don't sleep. Sigh.
Annndd.. That's it. Simple, right? Yep. That simple.