I appear more confident when I know what I am talking about in any situation.
I believe I have lived some--and I know what to do, and what not to do in certain situations. I believe this knowledge helps me to seem more confident.
"Saying what you mean, and meaning what you say" is very applicable in this situation.
Overall, conveying the sense that you are a stable person emotionally, and that you have some idea about what you are talking about, is important in somebody exuding confidence.