My own experiences with visualization have shown me that I perform best when I relax and allow the image to build in my mind. I do not force it, but instead let it start to take on a life of it's own. Rather than push my mind, I try to lead it. If I want a detail in the image to become sharper, I might ask what more information might be at that place in the image and then allow my mind to construct it for me. I hope that this helps a bit.
