Here are a few simple tips to improve your home's indoor air quality:
Avoid leaking basements which can breed mold in your walls and carpet.  Routing water runoff with gutters is an easy way to keep water away from seeping in the home.  Make sure gutters are clear of any leaves, sticks and other debris.  Dehumidifiers are also a great way to regulate the humidity.
Service your HVAC unit to make sure dangerous gasses are not leaking out.  Newer units usually have fewer problems than older ones.
If you have a garage, make sure you have a weatherproof door between the garage entrance and the rest of the home. This will keep gasoline and other odors out. Never leave a car or lawn mower running in an attached garage or shed.
In the kitchen, if you have a gas stove, make sure you have an exhaust fan with a hood to exhaust odor. Opening a window or door when cooking will also help remove gas fumes.  Have all fuel burning appliances inspected by a professional yearly to avoid carbon monoxide poisoning.
Open windows and leave doors open between rooms to help ventilate and circulate air.  Keep your home smoke-free. 
Keep your home as clutter free as possible.  Storing toys, books, etc. in closets or storage bins can help eliminate the amount of dust that accumulates.  Cleaning regularly will improve the air quality.  Wash all bedding in hot water to reduce dust mites and other allergens. 
Exhaust fans in bathrooms are very helpful to remove moisture, and chemicals from cleaning products. 

These easy and inexpensive ideas will hopefully make for safe and happy homes.