Jeff Sexton got Patrick Sullivan, Jr. to list the top things not to do when conducting a usability test. The top 6 list should serve as a good reminder for the UX Designer who does the occasional user test.
1) Never ask, “What do you think about this?”
2) Don’t feed the tester with your question.
3) Don’t let users be the designers.
4) Don’t let the statistics fool you.
5) Don’t get discouraged.
6) Don’t try to test too much at once.
I tend to actually ask #1, but using the first impression test, i.e. showing a page on the screen for 5 seconds, taking the screen away, and then asking something along the lines of tell me what you think the site is about, what do you think you could do on that site? I also liked the suggestion in the comments of giving the user an easy dummy task to start in order to warm them up. Head over to the article to see what the list of 6 "don'ts" refers to.