First get every girl involved in the house... or else it not going to work as well and it wont be that sisterhood team effort you need.
Put flyers up all over campus promoting your group.
Plan events or even just random open house for girls who might still be interested in joining. We had a "Friends" Night where we just invited girls over to watch tv and hang with us. Take em Ice skating, or out for ice cream. Some doing COB's on my campus even put ads in the news papers. Get girls to call their friends who might be interested. Talk to people. Sometimes a big group of us would have lunch in our student union wearing letters amd stuff to get people to notice us so we could recruit. It works. Take interested girls out for coffee dates and lunch dates like we did. If you can, maybe get a sorority (that you're close with) that's not doing COB and has enough girls to help you guys out.
In all of this though, remember its quality, not always quantity that matters. Get girls who will better your sisterhood.