Measure the floor space of the room to ensure the rug will fit. Place a piece of masking tape on the floor where you envision one corner of the rug being. From the tape "corner," measure out to where the next corner will be. Continue until you have four corners.
Make sure your measured area is away from floor vents and doors -- be sure to allow enough room for doors to swing open without getting caught on the rug. If you wish to accent a hardwood floor, be sure to leave several inches of space around the outside edges of the rug.
Place a non-skid rug backing on the floor before laying down the rug. This can be found at most home stores and can be cut as needed to match the size of the rug. This will help keep your rug from sliding -- a hazard in high-traffic areas.
Consider using furniture coasters if you place heavy furniture on top of the rug -- this helps prevent damage. If you place a table and chairs on the rug, in a dining room, for example, be sure there is enough rug space to move the chairs back from the table without the chair legs moving off the rug.