There are numerous strategies, products and ideas out there on IAQ. I'm going to add a little to the topic here and focus on air conditioning, since it is that time of year (finally!).
With residential air conditioning, whether it be a central unit, a 'window shaker', a portable unit, or a ductless split system, it is important to remember that as the air in your home is cooled part of the process involves the removal of moisture from that air. It is tempting to open up your windows after the sun goes down to take advantage of "free cooling", but if the humidity is high, it just adds to the load in the house that the air conditioner will see the next day when it is turned on again. There's nothing wrong with a little fresh air - in fact the more the better (as long as you don't live next to a freeway, factory, or a fastfood restaurant - but then I guess then that that air wouldn't be fresh), but just be cautious on those cool rainy days or evenings about opening up those windows and letting all the humidity back in that your air conditioner worked so hard to remove. Remember - dry air feels cooler than humid air, potentially allowing you to keep your house at a warmer setting in the summer - and saving you money with less AC use.
Of course as always, keep your humidifier on low (or off) during AC season, and keep those filters clean!
