Play with lighting: Lighting plays a major role in any interior space, hence you should make a creative use of it. I mean artificial and natural lighting can be used very effectively in an interior space.
Since Sun is a major source of lighting during day time, this can be used very creatively for creating "shadow patterns". Light which penetrates through windows can be interrupted to create shadows on inside walls and thus create an interesting feature.
As far as artificial lighting is concerned, and spot lights can be used to enhance certain areas of the space, by diverting attention. Many creative designs of decorative lamps and are available today at reasonable prices.
*Wall Paintings: Paintings is another excellent way to give a personality to a space. Depending upon the nature/subject of the painting the mood of an interior space can be altered.
Depending upon your likes and dislikes many beautiful wall paintings and posters are available in local stores, art galleries, etc...
Photographs of your family members is a great way to add a personal touch to your room.
*Wall Hangings: are a great way to add interest and are used as fillers on walls. A wall hanging can be just an art work or can even serve a purpose such as a letters holder.