Last minute sweeper tips?

i play on jv and it's our first game tonight. my stopper isn't all that dependable, and the ball gets by her a lot. i've always played flat back four (with a couple exceptions where i did play sweeper with a really good stopper, so i hardly ever touched the ball). i'm really nervous because if it gets by me, it's a breakaway. any last minute tips? we've only had practice a couple weeks and i found out i'd be starting sweeper on monday.