Clarity suggest I tell you why I know what I'm talking about before answering your question... so...I have run dozens of these tests from early stage prototypes to global research projects. Here are a few generic tips for you:

Flow:
1. Break the ice
2. Open ended exploration
3. Hypothesis testing

* Avoid using the word "Would" as it generally results in inaccurate responses.
* Reassure them that you want brutal honesty.
* Start by warming them up. Ask them to tell you about the context in which they used your product. That will usually break the ice and get them spilling the beans without realizing it. In other words, "What was going on in your day when you were using this?"
* This weekend, focus on a few key assumptions you want to learn about. Frame a few open ended questions around those.
* "Did this solve any problems for you?"
* "Did you enjoy anything in particular?"
* "Did anything frustrate you?"
* "Would you tell your friends about this? If so, which part? If not, why not?"
* "Was there anything missing?"
* "Any ideas on how to improve this?"
* Then, enter in with some more pointed questions.

Hope that helps!