At my school, you could only attend 2 pref parties. So I went to my first one (thought it was kind of corney and lame) Then I went to Sigma Sigma Sigma, now, my sorority.
What I've learned since then is that, at least at my school, if the sorority invites you back to pref, they want you. But being the unknowing PNM, I wasn't convinced. However, the girl who pref'd me asked me how my weekend had gone, what other party I had went to, what was done at the other party, and then asked me which way I was thinkign about going.
I told her I was divided even though I secretly wanted to be a Sigma, I waited for some sort of flag from her. Sure enough, I got a few. She talked about how other girls felt about today, and what other girls who had talked to me had said. All the while saying, you've got to go where you belong. When I, she said "I hope to see you tomorrow." Which to me, was the best way to go about it without saying, "You are getting a bid" or "You shoudl come here"