1. Really knowing your songs.
2. Really loving the songs you sing.
3. Really liking (or respecting)the people with whom you sing.
4. Because you know and love your songs and like the people you sing them with, being comfortable, confident and loose.
5. Knowing as much as you can about the history of the songs you sing, enough to be able to tell people about them.
6. Showing up early or on-time for your sound check. Warming up before you start.
7. Writing out a set list with a few options so you can be flexible if necessary.
8. Not worrying (or thinking) about anything else while you're performing like "Is my brother, Liam, going to remember to turn on the tape recorder. I really need a live demo tape. Etc."
9. Getting plenty of rest before you perform.
10. Doing it a lot.