In no particular order...
Get rid of toxic "cleaners"! This especially includes what you're washing your clothes and bedding with since your skin is the largest organ you have and it does absorb what you surround it with.
Change your HVAC return air filter seasonally. This filter is the lungs of your home and definitely needs to be changed regularly. Not changing this filter can also put more strain on your HVAC system and decrease its efficiency.
House plants! Absolutely. There are plenty of lists that are just a Google search away that can help you find the types of plants that fit your needs.
Verify that any indoor gas appliances are not leaking. Your gas company should offer this test for free. If you smell gas even though the test comes back negative, have it tested again!
Electromagnetic fields can also cause many problems, especially for infants. So remove or reposition any electronics away from where people are sleeping. Using a non-metal bed frame and a mattress without metal springs will also help cut down on electro-pollution.
Ron and Lisa of GreenNest.com are all about helping homeowners do just this. They just finished a new book that's a perfect tool for you to accomplish this goal, too!
For more information:
Read Evan's answer to the Q&A "Do I need an indoor air quality test? How much will it cost?"
Check ourhome air quality Q&A to see what questions other homeowners are asking.