I've always struggled with making my apps look decent. In my head I can picture how it should look, but when it comes down to it, I often end up with stuff that looks like a dog's breakfast.
Here's the cure:
That post in particular helped me understand how to use Blend to turn down the ugly; but Corrina's blog in general is good for those of you who use Silverlight.
Lose the borders on your ListBox and your app will already look way better :)