Don't be afraid to experiment. Take advantage of the short iterations of XP to try new ideas, but give them long enough to make a good judgment about how they worked.
After the first iteration, you'll have regression tests to run, in addition to writing and automating new tests. This is where you'll see the payoff of automation.
Don't worry if you didn't get all the tests from the previous iteration automated. Make the story estimates for the second iteration bigger and catch up.
Estimate what it will cost to fix a defect and let the customer choose whether he wants fixes or new features developed.
Prepare your customer for the potential chaos that can erupt right before a big release. If you're diligent with your XP practices, the release will feel anticlimactic, but prepare for the worst.
