You have to deal with the fact that if everything works well for a user, she will not provide a feedback. Because she don't care. Because she has nothing to say.
There are several approaches, and no seem to work well.
For example, http://digitalwpc.com/ provides a link for a feedback. It is not targeted for people who are experiencing UX problems (because the form you have to fill is not intended to provide you help of further support), but I'm pretty sure that it's only used by people who actually have UX issues with the website. Personally, I used it to tell them that their keynotes cannot be watched full-screen on dual monitor configuration; if everything was fine, I would never use their feedback to actually tell them that their website rocks.
Another example is a more intrusive feedback. For example having a Logitech keyboard and mouse, I were asked several times for a feedback. Again, I used it to highlight some user experience flaws with their keyboard/mouse combo.
This being said, both times I also provided positive feedback. In the case of WPC, I selected the highest rank for content, because I really appreciated it. In the case of Logitech, I also selected highest marks for the points I appreciated the most, and even added a few points about what I enjoy about the keyboard.
It means that:
- You get a feedback mostly from the people who are experiencing some issues with your product,
- But this feedback doesn't have to be only negative if you invite the users to broaden their feedback.
If you provide just a text field asking people for their feedback, they will talk about their concern, and their concern only. If, on the other hand, you invite to talk about the general experience, to evaluate precise points, the same users may provide additional, positive info.
For example, if your desktop application is horribly slow, but with a nice design, easy to use and works on every device including smartphones, if you ask in the feedback to evaluate the performance, the design, the ease of use and the interoperability, you'll see some very low results for the first point and lots of negative comments about how slow your app is, but you'll also see more high ratings for the other three points.