I reckon it is kinda like hunting game. Once your eye and your mind knows what to look for, you naturally start seeing it. My first rabbit hunt of the year is always tough because all I see are the runners. Later in the season, after I have hunted them a while and they should be much more wary, I start to see them sitting and holding tight. It's the curve of their back, the black of an eye, the hint of an ear.
Maybe if I knapped, I might start finding arrowheads faster, too. *sigh* Yet another reason for me to come over to the dark side!