Publicize the charity-based work in which your sorority participates. Whether a one-time event to raise money for your chosen charity or volunteer hours given on a regular basis, promoting these activities will present a positive image of your sorority.
Take academics seriously, both individually and as a group. If half the sorority is absent from class the day after a big party, this just goes to further set it in people's minds that partying is a sorority girl's first priority.
Keep in mind, particularly when wearing your letters, that you're representing your sorority at all times. Making a public spectacle of yourself at a party gives the entire group a bad reputation.
Think of ways to present a positive image on campus. Volunteering your time to help incoming freshmen get acclimated to campus life and find their way around is a good way to promote yourselves and maybe even get new members.
Encourage members to participate in other campus organizations other than the sorority. The more people get to know the members of your sorority one-on-one, the better you'll be able to prevent sorority stereotyping.