Des Traynor writes about how to get valuable feedback from your clients. He writes from a designer perspective, but the key points just as well apply for software developers, not only when talking to customers, but also when doing audits and reviews:

Every time in my career that I’ve gotten useless feedback it has always been because I hadn’t asked for anything more. Attaching a file to an email and asking for any thoughts the client may have is a sure way to get any thoughts that the client may have.

If you don't want to start a brain storm about whatever you have created, ask specific questions. When you create mock gui the questions might be:

• Do the colors and layout match your Corporate Identity?
• Are the most important elements easily accessible? How can we improve on that?
• Should we make the fonts larger, smaller or keep them as they are?

When doing a code review questions or tasks might be:

• Look for ill designed methods, that don't work on a single level of abstraction, or aren't properly named.
• Is there anything that looks like a bug?
• Is there anything that is difficult to understand and therefor needs refactoring or a comment?
• Is there anything that might result in performance problems?

It is also important to consider if you actually want feedback in order to improve the piece of work, or if you just want a confirmation that everything is Ok and you can consider it done. In the first case ask open questions: "What can we do to improve ...?", "What do you think about ...?" If you just want to get an Ok ask closed question, that can be answered with yes or no: "Is ... acceptable?", "Do you agree ...?"

Just in case you didn't followed the link to Des Traynors blog you mist the hilarious video he included in the post, so let me add it here.