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.