Seasonal allergies are the result of your body's overreaction to certain substances in the air at certain times of year. Also called hay fever or allergic rhinitis, seasonal allergies are what happen when you breathe in pollen or mold and your immune system thinks something harmful is happening. As a response, you produce antibodies to attack the "invading" allergens. The antibodies trigger the release of a swarm of chemicals, including histamine. Histamine is responsible for symptoms like sneezing, coughing, runny nose, sinus pressure and itchy, watery eyes.
The first step in dealing with your seasonal allergies is avoidance. You should try to stay away from whatever causes your allergic reaction in the first place. Trees tend to pollinate in the spring, grasses in the summer and weeds in late summer and fall. Mold releases its spores from spring to fall. While it's unreasonable to assume you'll stay indoors for months on end, you can try to limit your time outside when you know the pollen count is high; your local weather forecast should include a pollen count. Dry, windy days are worse for seasonal allergies than cool, rainy days. Have other people do outdoor tasks; if you have to work outside during allergy season, wear a dust mask to prevent allergens from getting into your body. Good air filters in your air conditioning and heating help keep the allergens from getting in your house and car, as do closed windows.
If you end up with symptoms or you know you have a high chance of them on a particular day, you can take allergy medications. Antihistamines, decongestants, nasal sprays, eye drops and cromolyn can all help reduce and relieve symptoms. Talk to your doctor about which drugs are best for your situation. In addition, you might want to consider immunotherapy -- or allergy shots -- as a long-term plan to reduce your reactions.