Several people have said this already, but it bears repeating: get a bed frame. I rocked the mattress on the floor for like 15 years before I finally got a proper bed frame and it really does make a difference in both the look of the room and your own mental well being. I got an inexpensive foam mattress with it's own built in "box spring" layer so I didn't have to get a separate box spring or anything. It's the kind they deliver right to your house rolled up in a box!
For me it was kind of like saying to myself "hey, you are the most valuable thing in this room so you don't belong on the floor".