Forgive me if I repeat some of what others have said......
Don't wear clothes that don't fit. Try everything on a couple of days before rush, and make sure your outfits aren't either too snug or falling off your body.
Don't dress like you're going clubbing. You're there to meet your potential sisters, not guys.
Don't wear clothes that are wrong for the season. You don't want to melt or freeze. But, if it's going to be hot out, carry a cardigan or wrap - you never know when you're going to walk into a party and find the A/C on full, and goosebumps are not a good look.
If you find you don't need it, a sister or your rho chi will set it aside and you can collect it at the end of the party.