What kind of airconditioning filters should I use? How often should I change them? There is always so much dust in my home>

You may get some better results with a higher quality air filter than you may currently be using.

I use Filtrete filters, they are very effective at the reduction of dust and allergens in the home.

It may be a good idea to check into having your duct work cleaned out as well.

Here is a link to a site with LOTS of options for good filters.

I change mine at a minimum of 90 days, but normally about every 60 days.

Been using polyester filters here for 30 + years. The big problem with many high efficiency filters is they can become to restrictive when partially loadad with dirt causing system performance problems. If you change them every month that is OK but the poltester with a spray on coating will give a good two to three months service. Whatever you do forget the fiberglass filters. The glass breaks off the filter and gets into the airstream of your house. Then you get to breath them in which is not good.

Example

Polyester Disposable Air Filter (http://filtera.stores.yahoo.net/poldisfil.html)

When I run across people that are using the cheap fiberglass filters, 79-89 cents each,the best I can do sometimes is advise them to change them every 30 days wheather they look dirty or not.