Many times being well put together and confident can go a long way. As, Sororitysock said, if you're wearing letters make sure you've got makeup on, hair looks good (even if you're having a bad hair day, put it in a neat pony, chignon or cute headband), nails aren't bitten down, legs shaved, etc. Maybe have a sisterhood event dedicated to getting glam? Give each other mani/pedis, facials, etc? Perhaps some members didn't have a mom or older sis at home to show them these things? As much as everyone will want to say looks don't matter, they do. No, you don't have to look like a Sports Illustrated swimsuit model, but giving off that glam vibe can make a big diff. Are there exercise classes at your school's rec center? They should be free to students (at least they were at my uni). Maybe encourage a group (no matter a sister's size) to go and have fun together. It's easier to get started on exercise with another person. Plus, you have someone to moan and complain with during the class or when you're sore later.
Also, get involved on campus, plan mixers with fraternities you all don't normally hang out with and that can help crash some of the stereotype. Try to win things like Derby Days and other Greek awards like best GPA, intramurals. No one can dog a sorority that wins everything or at least makes a big showing. Be the sorority known as the go-getters.