There are many effective ways to make your house and home smell nice, and stay smelling nice. There are really two sides to this: eliminating the source of bad odors from your home, and replacing them with nice smells.
Eliminating Bad Odors from Your House, Home or Apartment
There are many common sources of bad odors in the typical household or apartment. Perhaps the most common is the smell of trash -- particularly kitchen trash! Be sure to take the trash out often. Also, consider purchasing a trash can with a heavy lid that sits over the rest of the trash can using a lip to "seal in" the bad smells within. Also, consider throwing things that smell particularly pungent directly into a trash bag and then take it out, rather than keeping it in the house.
Another very common source of odors is pet-related items such as pet food cans, litter boxes, etc. Change these often to help to reduce bad odors.
Finally, dirty dishes in the sink can contribute a bad odor to a household. Consider placing dirty dishes directly into the dishwasher to avoid stagnant smells in your house or apartment.
Replacing Bad Odors in the Home with Nice Smells
In addition to the first problem, there is of course the issue of replacing bad odors with nice ones. Consider getting an activated odor producer, such as an air freshener -- particularly active air fresheners that are plugged in.
In addition, you might also consider getting an air purifier. These add fan noise, but can reduce the levels of dust and pet dander in the house, but the activated charcoal filter can also reduce actual odors directly by removing the trace chemicals in the air that produce the odor.