If it's not clean when it's shown, they'll naturally assume it will be dirty when they take possession. Look everywhere for dirt, and get rid of it. Check behind the toilets and taps, under appliances, on the furnace and water heater and on top of door and window casings. You know that fuzzy stuff that's stuck to the fans and cold air returns. Yes, this matters too.
Buyers tend to view a home just the way it is and conclude as follows.
This is the way this home is. If I buy this home, it will look this way when I live here too. Take some time to get your personal effects in order. Start packing now. Box up the things that are just hanging around collecting dust and neatly stack them away in the storage room.