I read a great blog post about cognitive overhead, and it reminded me of one of the simplest pieces of user research you can do on an established product.  I’ll quote the article:

Let people use your product, and then ask them to tell you what it does. They’ll think you are crazy for not knowing already, but what you hear can point to cognitive hurdles you’ve missed.

This is an awesome way to get some fantastic data about how your users see your application.  If their answers surprise you, either because they talk about things that you don’t think are important, or they miss things that you think are essential to your product, then you’ve got some problems that you need to address.