Upperclassmen have a harder time getting bids than freshmen do.
Not true at all actually. I don't go to USC but am very familiar with their Greek system (siblings, one parent, many friends all went/go there) and I know that even at the most competitive chapters on campus about a third of their pledge class is sophomores. For juniors is becomes very hard but because of their Spring Admit policy a sophomore, generally speaking, has an equal chance of getting a bid as a freshman (the only reason they wouldn't is if they had a bad rep on campus). But yes, recs and grades and all that are very important.
Since your first semester will be in the spring, use that semester to get to know sorority members through your classes, activities, dorm, friends, etc. There will be other women in your shoes because of the spring admit policy, but there's no better way to get an edge than to actually get to know sorority members. Just don't put all your eggs in one basket.