Just relax and be yourself. COB rush is a lot less hectic and intimidating than formal recruitment.
Definitely practice your conversation skills - the sisters do the same thing in preparation for rush.
You may want to put together a mental list of questions to ask in case there's a lull in the conversation (or in case you are asked the dreaded "So, do you have any questions about XYZ?"). Most of these should be about the sorority, though (as FSUZeta suggested) some of them can be chatty small-talk type questions about movies, books, etc.
Good luck!
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