Install sound deadening foam on walls and ceilings in the room that you would like to reduce reflections in. In most cases, sound deadening foam will include adhesive kits that are safe to apply to the foam without harming it. Sound deadening foam can be purchased from a variety of companies including Auralex, Foam by Mail, and Sound Proof Foam. To learn more about products from these companies, refer to the links below in Resource.
Add padded furniture, such as cloth couches and overstuffed chairs, if there is space in the room for such objects. These pieces of furniture will aid in the absorption of sound and help deaden the room.
Install carpet in the room to further deaden sound. If you do not wish to carpet the entire room, adding floor rugs will serve a similar sound deadening effect.
Install curtains to window sills. In addition to helping with sound deadening, installing curtains will help block sound from coming in and out of the window. The thicker the curtains, the more sound they will absorb.