Lisa Crispin and Janet Gregory have written extensively about this. They have written here and have written a book about it. (No, they didn't pay me to say this! I do know them and like them :-)
Their book is "Agile Testing: A Practical Guide for Testers and Agile Teams".
Elisabeth Hendrickson is another agile test author whom I highly recommend. She has a new book out, "Explore It!"
My guidelines: Automate the feature testing so you can explore around the features. As with all advice, your mileage will vary.
Start with these suggestions and you will be on the right track.