What are the ways to prevent sinusitis?

Keep the nose clear. Since the sinuses drain into the nose, when nasal stuffiness blocks this normal drainage an infection of the sinus can result. If you suffer from nasal allergies, an antihistamine can help to reduce that stuffiness, or a decongestant for stuffiness due to a cold. If problem persists, see your ent, allergist or family physician for help.
Allergy control. It is usually caused by some form of blockage of key draining pathways in the sinuses. The #1 reason for this are allergies. If you are prone to allergies & sinus problems, you should aggressively control your allergies with otc antihistamines, prescribed nasal sprays (steroid sprays and antihistamine sprays), prescribed oral medications, sinus irrigations, allergy avoidance, & allergy shots.