All you need is a halter, a lunge line, and a lunge whip. Personally, I've never used a round pen and free lunged, so I don't know what that's really like!
I have a 16.3 HH, nine year old Thoroughbred mare. She's sassy, to say the least.
I'm really calm when I push her out into the circle and I quietly cluck to get her to trot. I keep the lunge whip in the hand that isn't holding the lunge line in case she invades my personal bubble. From time to time, she'll kick her heels towards me, and a simple flick on the ground gets her to stop her behavior (she hates lunge whips/the noises/everything about them, from what I can tell). Also, a whip held up while you're lunging could provide a visual for the horse about where their space starts/stops and where your personal space is as well.
Just remain as calm as you can and try to keep her slow and relaxed as well. Don't let her get under your skin and upset you.