Here is a good article:
http://well.blogs.nytimes.com/2012/12/17/helping-children-gain-control-over-an-anxiety-disorder/?_r=0
For my DD, it helps to personify her anxiety - treat it like a person that tries to take over her thoughts and needs to be put back in her place. She will sometimes draw a picture of her anxiety (this person she has made it into) and will scribble over it or tear it up to remind herself that she is in control and even though anxiety is always there, she has the power to control it herself.