Commonly, if someone wants to add a fresh scent to a room of their home, they think of a synthetic air freshener from the grocery store. However, if you are thinking about this for the purpose of selling your home, this can be dangerous.
Increasingly there are more and more people that cannot tolerate synthetic perfumes--and if you have used one in your home, a potential buyer that is sensitive to these types of scents may not be able to get past the front entrance, or if they do, will almost for sure be turned off the home as they develop a headache or feel nauseous.
That being said, adding scent does allow you to stage your home on another level, and can be a powerful way to influence potential buyers so we don't want to ignore it completely just because the synthetic smells don't work for some buyers.
The first part of controlling the scent in your home is pretty obvious--make sure there are no negative scents. Empty every garbage can every single day, make sure the laundry hamper is emptied of dirty clothes, make sure the appliances are clean inside and out (especially front-loading washers which can develop a mildew smell that you may be used to but will be offensive to a potential buyer), and use natural/unscented cleaning products. A clean house smells best!
I also really like to open the windows wide & let in fresh air--yes, even in winter. You won't freeze to death from 15 mins of open windows, and if you do it enough in advance of your showings, then the house will have plenty of time to warm back up.
My favourite way of adding a mild scent to the home is an essential oil diffuser or room spray. Apart from the lovely smell, essential oils actually have many health benefits as well. For example, peppermint oil has been know to reduce stress, tension and headaches. While lavender oil is believed to have antiseptic and anti-inflammatory properties, it is also thought to be a great way to relieve anxiety and depression. All of these are exactly the emotions you want potential buyers to feel when they are in your home. A great place to purchase these items, and get great advice about their use--ie how much will be just right and how much will be too much--is Sage.
My last recommendation I have for adding a nice scent to your home is probably the simplest one. Buy some flowers--there's nothing better then taking a big whiff of a fresh bunch of roses - and you & your potential buyer will also be able to enjoy the view of a beautiful bouquet!