When developing a space plan, I always start with a floorplan.  If you don't have a floorplan of your room, take some measurements and use graph paper to draw up the room.  Looking at a floorplan allows you to get a better feel for the amount of space you are working with, since sometimes ceiling heights, furniture, and other items can make it difficult to perceive space while in the room.

I use AutoCad to create my space plans.  Using this software allows me to move furniture virtually before I ever lift a finger in the room itself.  With AutoCad I can measure distances for traffic flow, even spacing, and furniture placement.  If I don't like it, I simply edit it until I do.