To prevent vaginitis:
Keep yourself clean and dry. But doctors don't recommend vaginal sprays or heavily perfumed soaps for this area. Douching may cause irritation, too, and more importantly, could hide or spread an infection. It also removes the healthy bacteria that do the housekeeping in your vagina. Douching is never recommended.
Avoid clothes that hold in heat and moisture. Nylon underwear, tight jeans, non-breathable gym shorts and leggings, and pantyhose without a cotton panel can lead to yeast infections.
Eating yogurt with active cultures (check the label) might help you get fewer infections.
Condoms are the best way to prevent passing infections between sexual partners.
Get a complete gynecologic exam every year, including a Pap smear if your doctor recommends it.