I wrote a blog post on how to effectively improve your game by letting people playtest it.
The points I'm making are:
1. If you’re not embarrassed, you’re testing too late. Test as soon as you have something playable
2. The ideal tester doesn’t know you and has never played your game before. This is the best way to get an unbiased first-time player opinion
3. Watching a player while testing the game may skew your results, but is you r best shot at learning a lot in very little time
4. In addition to letting your testers talk freely about what they think, prepare a questionnaire so that you can ask all your testers roughly the same set of questions. This helps you notice emerging patterns.
5. Consciously decide on what actions you're going to take to address each feedback point you get (even if your action is to just ignore it).