Hello sister!! Come over to the ASA forum and introduce yourself!
Make sure that everyone in the chapter is involved with another group on campus - preferably one that puts them in contact with underclassmen. This is one of the best ways to meet prospective members.
Get your letters out on campus. Go to the dining hall for lunch - don't just go off campus or to the house to eat meals.
When you are looking at new members, try to get a cross section of ages - i.e. don't take all sophomores or all freshmen or in 2-3 years you'll have the same problem again. Do you have fall formal and then COB in the spring?
OTW is right - go to SisterCircle and get some suggestions there as well. If you need help with getting signed up there's a email address you can send questions to.