Stop smoking/take preventative (or protective) measures
It is impossible to avoid airborne allergens entirely, there are still numerous things you can do to try and reduce their effect on your life. Smoking is the biggest known allergy trigger so stopping would be the first thing to try and do. If you have pollen allergies then try and have someone else do your yard work while you stay inside on windy days or in mornings when pollen counts are higher (air conditioning helps even more). If you do have to go outside for yard work, wear a face mask to filter pollen and keep it from reaching your nasal passages. Shower immediately after working outside. Leave all gardening tools and clothing outside.
Set your living measures up to counteract your allergies
Plant low-allergenic grasses, trees, and shrubs. Good flowering plants would be the ones that rely on insect pollination because their pollen is too large and heavy to become airborne. The best way to reduce exposure to dust mites of course is to reduce the amount of dust in your home. Consider replacing any carpeting with wood, laminate, vinyl, or tile floors. Try buying leather furniture. Get a vacuum cleaner with a HEPA filter which will filter tiny particles. The bedroom is probably the worst room for dust mites. Instead of having blinds use shades instead. Don't use feather pillows or down-filled blankets. Wash all bedding weekly in the hottest available water.
If you're allergic to molds you will want to avoid areas that tend to harbor them. Moist, shady areas outdoors, greenhouses, basements, closets, bathrooms, old foam rubber pillows, upholstered furniture, garbage pails, places where fresh food is stored, mills, grain fields or bins, garden compost piles, etc. Open a window after a steamy shower or bath to allow the humidity to escape or install a fan in your bathroom.
Stay away from pets
If you tend to be allergic to a multitude of things, it'd probably be in your best interests to not get a pet (obvoiusly).
Photo Credit: Kaptain Kobold