I've thought about it just a bit more now, because something *has* been changing for me regarding that within the last couple of months (the last month has been especially noticeable). And, part of it came clear to me that, in my case, it seems to also be linked not *just* to the musical images (or preparedness) that I'm building through mental practice, but the practice of mentally practicing is building or expanding my ability to concentrate, as well. To be capable of thinking very very deeply and focusedly, even for 10 seconds, and in that time to cultivate an intense musical image, starts to build that mechanism who can think/be like that at all.
For a long time I've thought "we practice our attitudes about music and practicing while we practice our music" ... but, this is more defined than that. I'm still working on it, but my advice would be to practice utter focus in very small portions, building in portion size as you are able to accomplish it in the smaller ones. It's like a muscle you have to build and cultivate and develop, until all you have to do is "turn your gaze" and it's just sitting right there, ready to be called upon.