You can save yourself a lot of time and trouble by taking careful measurements and thinking through the following issues before you start drawing.
1) When measuring existing spaces, lay your measuring tape flat on the floor and measure room dimensions in several places, especially where furniture will be a tight fit. Don't assume your walls are parallel.
2) Before laying out furniture, look for projecting windowsills, and note the location of electrical receptacles, light switches, vents and thermostats.
3) When placing a desk or chair, ask yourself "What do I want to be looking at when I sit there?"
4) When planning an office, first determine whether the occupant's back will be toward the door. Some people feel very strongly about this. Other design decisions will flow from this basic choice.
5) To reduce glare, try to avoid having computer monitors directly facing windows.
6) Leave enough empty space in front of filing cabinets to fully extend the drawer, plus at least 18 additional inches (1 foot, 6 inches) if the drawers will be accessed by a person standing in front of them. *
7) Allow adequate working space at desks or cubicles. The distance from the working side of a desk to the nearest wall or furniture should be at least 42 inches (3 feet, 6 inches), although most people find 54 (4 feet, 6 inches) or 60 inches (5 feet) more comfortable.
8) The walkway between a piece of furniture and a wall should be at least 30 inches (2 feet, 6 inches) in a residential space (36", or 3 feet, is preferred). In a public space it should be at least 36 inches (42", or 3'6", is preferred). *
9) To get a feel for the width of a walkway before placing furniture, put a tape line on the floor to see how different dimensions would feel.
10) When placing furniture on a residential plan, remember that most furniture will actually sit 2 or 3 inches from the wall.
11) Don't line all the furniture up along the walls. Break up spaces by placing pieces out in the room. Setting rugs, sofas, or other furniture at angles can help avoid an overly rigid feel.