We have a 1430 sq ft ranch syle home built in 1965 that is surrounded by farm land. We have been here for almost 1 yr. When we moved in, we had the ducts professionally cleaned..( a vacumn with a spinning brush) and have been replacing the filters with a $16.00 3M, Merv 12 every 3 mos. My husband has allergies. Is there anything else I can do to have cleaner air in the house. I dust constantly but it just keeps coming back. Should we change the air filters more often or different ones..the "professional" suggested we buy expensive,washable filters..like $200. Would others be better? Also, does the air travel down from the furnace through the floor vents and then up through the intakes on the ceiling. I hope I am installing the filters correctly..which indicates how to install based on the airflow. Any help or suggestions?

The 1-inch filter will not do the job. Have a duct or furnace repairperson install a 5-inch pleated air filter compartment and filter:
http://www.oxyclean.com/cartridgefilters.html
http://www.oxyclean.com/spacegard.html

Then leave the fan on low speed all the time, switching to the adequate speed for air conditioning or heating.

Dust is just one of your potential problems for allergies. Can be affected by everything from materials used in the building (OSB, carpets, paints, etc) to the quality of the indoor air. Windows are a potential problem as different types allow varing amounts of air infiltration (leaks). Also the basic construction can allow many air leaks (use of fixtures that allow air infiltration, no house wrap, various 'holes' in the 'living envelope', etc. A good filtration system is a good idea but may not solve all your problems. Here is a web site of the Mayo Clinic that addresses many of these problems: http://www.mayoclinic.com/invoke.cfm?id=FL00012

PS: We are in the process of building a home and we suffer allergies also. Are spending thousands of extra dollars to eliminate OSB in subflooring and cabinets and installing only hardwood & tile on the floors. Also venting (bathrooms & kitchens) to the outside is an additional costs. We have also specified forced air furnace with 'sealed burners' that gets outside air to burn and exhausts the burned products directly outside. Another added expense is the filter systems and a whole house heat recovery ventilator that brings in fresh, filtered air and exhausts the stale (polluted) indoor air while recovering most of the heat/cool from the exhausted air. The windows are being upgraded from double/single hung to casement and awning types that seal out the wind (and dust) better. Also applies to doors that seal better. Allergies are no fun and the fixes for your home are not inexpensive.

I live in Oregon in the grass seed capitol of the world! Allergies are a huge issue here! If you have a forced air heating system, I would look into a whole-house electronic air cleaner or an ultra-violet air treatment system. Honeywell has a great line of air cleaning products to help with just such problems: http://content.honeywell.com/yourhome/eiac-air_cleaners/air_cleaner.asp