At UCLA, upper-class quota is about one THIRD of the total pledge class. I'd have to go back and look up actual numbers, but I think it was something like 68 New Member class total with about 20 upperclass new members in that total. Not sure there's a higher % at any other school. UCLA has a large number of transfer students, so most chapters take the full amount of sophomores, juniors & even seniors.
Grades are very important. If you have less than a 3.0GPA as an upperclass PNM, your choices will be limited.
Most women you talk to probably won't even ask why you didn't join last year. If they do just tell the truth, then say that you're ready to join Greek life.
Tell your friends who are already in sororities that you're going through recruitment and that you're really excited about it. Keep an open mind, practice your conversation skills and maximize your options.
Here's a guide of what to wear:
http://npcbruins.com/what-to-wear