Purchase sound deadening foam and install it on your walls and ceilings. This foam can be used to spot treat the room to effectively deaden sound reflections. A few websites that sell this type of product include Auralex.com, Foambymail.com and Soundprooffoam.com.
Hang thick curtains over the windows in the room. The thicker the material, the more sound they will deaden through absorption.
Carpet or add area rugs to a room with wood or tile floors. Doing so will help absorb sound reflections that bounce off of your floor.
Add padded furniture and pillows to the room. These types of items are functional, but will also help to deaden sound in the room.