According to Zen Habits, the trick to successfully decluttering a room is finding a large area to serve as a working and organizing space. Then, you can:
"2. Start on one side of the room and work to the other.
3. Do one drawer or shelf or spot on the floor at a time.
4. Pull everything out of the drawer or shelf (or section of floor or what have you), and pile it in the working space. Clean out the drawer or shelf."
For the full approach, click on Zen Habits above.