[:.britt.:]

Patence is key.
You may wait around for hours after the show depending on what she's doing, and where exactly she has to go(or wants to go)that night and/or the next day.

I mean, if your seeing her play at a place where there isn't thousands of people, but maybe hundreds you'll have a better chance seeing her from my expirence at concerts(I can't confirm that's how it works for Avril because I have never seen her live). But again, as someone said 'be at the right place at the right time.'

Try to be there early, and wait around late.