Disclaimer: I'm no lawyer, so I would advise you seek the advice of someone who is qualified.
Now that all the legal mumbo-jumbo is out of the way, here's a sample template that you can use if you like. I've found that most clients appreciate that it is relatively simple and straightforward.
I've highlighted the areas in red that you'll want to be sure to fill in. But obviously, you can tweak any of it however you like.