When it comes down do it, really the only technical difference between a prelim and an outround is more judges, more audience members, and sometimes a bigger room. The rest of the round functions the same.
Because of that, the biggest change you have to make is to take control of the room with your presence. I know this sounds trivial, but having watched, judged, and debated tons of outrounds, I am convinced it is key. Interestingly enough, taking control of the room does not mean being more aggressive or determined to win. Instead, it means projecting more vocally, using humor and wit, using bigger hand motions, and smiling when appropriate. A joke or witty comeback in CX can immediately establish that you are completely comfortable debating in the round and that everyone should pay attention to you.
Example: NCFCA national policy final in 2011. There were four guys in finals, and the round was pretty dry. The negative team was arguing some good arguments, but was very serious in their approach. The aff team had a little more energy from the beginning, but still, no one had really taken control of the room. Then the 1N got up to CX the 2A and intensely asked him what these certain Russian scientists were using their personal pay for, and the 2A threw back this great response that was something along the lines of "That would be up to them...I suppose they could buy some of those little Russian dolls if they wanted." It was no big deal, but everyone in the auditorium laughed, and immediately we all felt more engaged with the aff team then we did with the neg. Aff won the round, largely on argumentation, but also clearly on their ability to gain control of the room.
Kara and I had something similar happen in our national policy final in 2012. When one of the aff speakers CXed Kara after her 1N, she gave a very funny response (albeit, somewhat unintentionally...) that caused the whole auditorium to laugh. You could feel that the control of the room had shifted toward us.
I have never seen you speak, but I am going to guess that maybe you are not the most assertive speaker? Being a girl, I can understand that being assertive in a round can be difficult, especially vocally. However, especially if you are getting comments about not smiling, taking control of the room is probably where you need to start. The lectern is your friend. Make it your happy zone! Pretend you are William Wilberforce or something...
SMILING works in every round. Happy smiles, regretful smiles, sympathetic smiles, jubilant smiles: all of them work great.
By the way, you can practice all this in prelims--never hurts.
_________________The "Frazer" of "Frazer/Stivers" ; now an alumni. Buy Ethos Sourcebook. Unmatched quality, ethics, and quantity. http://www.ethosdebate.com/Go to an Ethos Camp this summer--one of the best investments for your debate career.
Praise the Lord, O my soul!