Summarize this article:

For instance, if you have a very childish looking dresser, position it somewhere in the room that hides or deemphasizes it. Consider revamping childish furniture, as well. A new coat of paint in a different color can make a world of difference on even the most insignificant of items.  Always get permission before you permanently alter any furniture in your room. If possible, try to get pieces of furniture that match each other or are part of the same set. Add vibrant accent rugs and well-constructed bean bag chairs to your floor to provide an additional pop of color and to give the space a little more texture. Stack a few bright throw pillows on your bed and nestle a few into the bean bag chairs.  Stack a few warm colored blankets on your bed in the wintertime for a cozy look. Try hanging colored lights (like Christmas tree lights or Chinese lanterns) around a window, your bed or a bookcase. This can add a fun dimension to your room. Fold blankets and place them into a basket for a neat way to store and display them. If you have a nice piece of furniture, such as a bookshelf or desk, move them around into the more visible areas of your room. Clean up and organize your desk or bookcase so that it looks its very best. This will give your room a much more mature vibe. Make sure to declutter the dresser and bookshelf and only display your nicest items on it to make it seem more grown up. While it usually depends on the size of your room, your bed is probably the current focal point. The traditional place for a bed is on the wall opposite the door, so this would be the most mature looking choice. If you have a nice bed frame and headboard but your bed can't be seen from the door, move it so that it becomes the focal point.  If you aren't sure, go with the arrangement that makes your room look the least cluttered. Open spaces are streamlined and more adult-looking than a cramped room. If at all possible, don’t put your headboard against the same wall as your bedroom door, since this can have a disorienting effect on anyone who enters.

Summary:
Move or revamp the less mature furniture. Create texture and layers. Make a nice desk or bookcase the focal point of the room. Reposition your bed.