Get dust, flies and moths out of light fixtures. Get rid of any odors. Get rid of ashtrays. Use an air freshener if the house does not smell right. Eliminate the smell of dogs, cats, kids, spicy food, etc.
Odors are hard to explain away and are always noticed.
Clean the windows outside and inside. Find the best real estate agent in your area from an unbiased reliable source. Remove clutter from the kitchen. Keep the sink clean, free from dishes. Clean the refrigerator, degrease the oven.