If possible, ban cell phone use completely during game play. Pulling out a mobile phone while another player is sparring with an angry wizard will destroy the atmosphere in the room pretty fast.  If natural light spilling in from nearby windows is distracting and/or conveys the wrong mood, cover them with blankets. Set up the game in a room that doesn’t have a lot of other things going on. An empty attic, for example, would be an excellent place to set up. Dim or turn out the lights and use candles. If you’d prefer to avoid possible fire hazards, try repositioning the room’s light sources instead. For example, you could move all the lamps into the next room and allow just a little of their light to spill into the game room.  You can get even more creative by throwing patterned and/or colored scarves over the lampshades. As the plot progresses, make sure to change up the lighting to coordinate with new scenarios and settings. Music is one option, although most contemporary music would throw off the fantasy setting completely. For a no-fuss solution, stream an internet radio station that plays classical, baroque, or Celtic music.  Using non-music sounds can greatly enhance a fantasy setting, as well. For example, you could play a CD that features nature or ocean sounds to intensify jungle or maritime settings and scenarios. Put computers on the other side of the room, out of everyone’s direct line of vision. Have the game master sit in a chair that positions them above the other players. You can also use room elements to mimic setting characteristics. For example, in anticipation of a sea scenario, you could set up a fan in the corner of the room to mimic an ocean breeze.

Summary:
Turn off the TV and minimize all other distractions. Change the lighting in the room. Use sound to enhance the atmosphere. Rearrange room elements to strengthen the atmosphere.