Scale of the home/room -- buy furniture to the scale of the house. The bigger the house or room, the larger the scale can be that you use to decorate the home with.
Lighting - choose paint colors and fabric colors that will be appropriate to the lighting of the room in daylight and in the evening. Paint chips will vary according to daylight. Paint a sample square on the wall and check the lighting and color of this before you paint your walls. This could save a lot of time if you made a mistake.
Furniture placement -- make sure that the flow of your home is comfortable and creates great conversation areas.
Feng Shui really helps. You want to de-clutter as much as possible and follow the basic principles of good feng shui.