It may pain you, but remove personalized things from the immediate viewing area. The refrigerator should be clear of pictures, the walls clear of family photos. This helps a buyer to better imagine your house as 'theirs'. Tell yourself and your family that the new home can be personalized to your heart's content, but for now, a more neutral setting is needed to produce the effect you want.
There are many specific ways that your property can become prematurely aged but most of them fall under two categories: lack of maintenance and lack of cleaning. Maintenance includes the upkeep of parts of your house like gutters and furnace, ensuring that those parts work. Cleaning covers more of the day to day vacuuming, shampooing, scrubbing, etc.