Buy some nice poster, and get some frames from Target or another big box store to hang on the walls. The one thing I notice about so many rooms is how blank the walls are. Art work of any sort is what adds personality to a room. Allposters.com has a huge selection of posters, photos and every sort of print you can think of at very reasonable rates. Upgrade the paper if you can afford to. Their shipping is a bit slow as I think they print them after you order them. I've got several photos from them that I've framed, and am very happy with.

Another great way to decorate is to shop garage sales and thrift stores. It's amazing what people give away that's still in perfect condition. I have 3 bookcases in dining room that I needed things to fill the empty shelves so I kept my eye out when I'd go to the thrift store over the years, and most of my knick-knacks are things I found there for under $5.