Check all of your windows to make sure they open and close easily. If not, a spray of WD40 often helps. Make sure there are no cracked or broken windowpanes. If there are, replace them before you begin showing your home.
Do the same things with the doors - make sure they open and close properly, without creaking. If they do, a shot of WD40 on the hinges usually makes the creak go away. Be sure the doorknobs turn easily, and that they are cleaned and polished to look sharp. As buyers go from room to room, someone opens each door and you want to do everything necessary to create a positive impression.
For those who smoke, you might want to minimize smoking indoors while trying to sell your home. Actually, it is best to move smoking outside. You could also purchase an ozone spray that helps to remove odors without creating a masking odor.
Apologies to pet owners, but pets come with odors. You may have become used to them, but they are immediately noticeable to those with more finely tuned olfactory senses.
For those with cats, be sure to empty kitty litter boxes daily and use plenty of baking soda. For dog owners, keep the dog outdoors as much as possible, even those adorable lovable little dogs. You might also try sprinkling carpet freshener on the carpet on a periodic basis.