Avoid contact with people who have colds and other viral upper respiratory infections. If you do have contact with people who have these infections, wash your hands often, especially after being in contact with those who are infected.
Avoid cigarette, cigar, and pipe smoke in your home and workplace. Smoke causes and further irritates inflamed membranes in your nose and sinuses.
If you have allergies, avoid the things that trigger your allergy attacks. Consider talking to your doctor about immunotherapy, such as allergy shots. For more information, see the topic Allergic Rhinitis.
Avoid breathing dry air. Consider using a humidifier at home and work to increase the moisture in the air.
Putzing in the garden is nothing less than therapy. It's even good exercise, if you exert enough effort. But the sneezing and stuffy-headed feeling that lingers afterwards -- that's the downside of gardening with allergies.