This issue is more common than you may realize, and there are few errors more egregious than showing a dirty home. The first thing you need to do is get your personal effects out of the home (you can leave large furniture like beds and couches). The second thing you need to do is scour the entire place from top to bottom. If you don't have the time or energy required to get your home looking new again, then hire a professional service. Seriously. Do it.
Odors are a huge issue; they're like dirt that you smell. Usually, opening the windows during the cleaning process will remove any odors. If they persist, you need to track down the source. Pets stinking up your carpet? Replace it. Musty smell coming from the vents? Get an HVAC technician to take a look and make sure there isn't mildew or mold.
A fragrance is the opposite of an odor, but it can have the same effect. Whether you love the scent of lilacs, lavender, or cigarettes and motor oil is entirely up to you (hey, I don't judge). However, people react to fragrances differently. If a person enters your home and find the fragrance unpleasant, they'll associate that unpleasantness with your home and are far less likely to place an offer. You want it to smell neutral and clean. You want it to smell “new.”
If you're selling an older home, you may want to take a look around and see if there's anything that seems dated which you can easily replace. You'll want to look at ceiling fans, light fixtures, doorknobs and even the little knobs on your cupboards. Things like this can usually be replaced pretty inexpensively, and will help give your home a more modern look, which, again, will help give it a feeling of being “new.” Guess they just don't make things like they used to, eh? No. They don't.
People are all different, and buyers will like and dislike a variety of features in your home throughout the course of the viewing. The items listed above, though, are pretty much universal turn-offs that you'll want to avoid at all costs.