Fight the stereotype. As KSUViolet said, even if a few people in your chapter are size 10 or 12, "tent talk" will be "ZOMG they're the fat sorority." (Believe me, I'd kill to be a size 10 again.

) So, represent, and look good. Have a letter day once a week, and encourage your sisters to wear letters on one or two other days during the week as well. When wearing letters (and even when not), encourage your sisters to look like they didn't just roll out of bed - you don't need to go all out and do full makeup, hair products, and jewelry (ok, except on pref night

) but at least run a comb through your hair and put on some lip gloss. And have a day, from time to time, when you wear your badges/pins and dress up. You will catch people's eyes when you walk into lecture wearing a blouse, slacks or a skirt, and tasteful makeup, jewelry, and hairdo, and everyone else is slouching in wearing jeans and a t-shirt.
As for recruitment, informal is a different animal from formal. With formal, you have short parties, lists, bid matching, a lot of rules, and a lot of stress. With informal, you can (and should) take the time to get to know a PNM in a low-key manner. Have a few sisters befriend her before you even get into selling your sorority.
All the best to you and your chapter.