While I'm not a fan of the residue left behind by candles, wax warmers help to bring a lovely scent to the home without the side effects the smoke will have on your lungs. If you are looking for a more natural approach, consider a simmer pot or essential oil diffuser. Of course these can be a quick fix before company arrives, but by utilizing these solutions on a regular basis, you will help your home to always smell pleasant even when the unexpected visitor swings by.
While these methods can "mask" unpleasant scents, it is also important to address them at the source. Clean your garbage disposal, don't let laundry sit in the washer long enough to adopt the cursed "mildew" smell, take out the trash regularly, and wash dirty dishes instead of letting them accumulate for days. If you have a pet, consider using a pet deodorizer on carpets, furniture, and pet beds frequently.