I have had plenty of opportunities to wear civilian clothes. Between passes, trip sections, retreats, ballroom dancing classes, and CAS jams (they're just dances), I have worn civilian clothes many, many times. I would recommend bringing at least 5 or so outfits with a little range in niceness (keep it pretty casual though). Pack a suitcase with that stuff and have your parents bring it up to you on A day. When you go home for Thanksgiving, change some of it out for more winter-appropriate clothing. When you go home for Spring Brake, change some of it out for more spring/summer-appropriate clothing.
If you don't want to bother with civilian clothes, at least bring some jeans, a shirt you like, and some sort of jacket/sweater. You can even pack that in your bag you bring to Beast and leave it in storage until the end of Beast, but I would probably wait until A-day. Long story short, you can always borrow clothes from other plebes or maybe even female team leaders in your company.