In our rush workshops, we did a lot of one-on-one rushing practice. Often an older sister would act as the PNM and have some weird personality quirk to act out, and a younger sister who'd never rushed on the other side would be the sister rushing her and trying to deal with her quirk. (One year I had to act the role of a slutty PNM, which is so unlike my personality that I fell completely out of character and just started laughing! So much for my acting career, hehe
)
We practiced body language, smiling constantly (ow), and steering small-talk conversations toward the sorority ("Oh, so you're a math major? Well, our sister Susie here is also a math major, and part of our academics program is blah blah blah...")
Like sororitygirl2 suggested, arm your sisters with the key facts about your sorority, including financial obligations, and with some questions to get a slow convo going or get a shy/nervous PNM to open up.
One thing that really bothered me about a particular sorority when I was a PNM, was that the sisters always, ALWAYS, asked "So, do you have any questions for me about XYZ?" I didn't have one convo there where I wasn't asked that. I felt very put on the spot to come up with something to ask, particularly once my "pre-planned" questions had been exhausted!
HTH