If your eye catches something, your mind almost stops itself with a spike of ouhh I like that. Then follow through on this emotional reaction, stop and take the shot. In fact my tip is really stop, evaluate the shot, take a few and don't rush. Often I've just grabbed a shot without having the courage to really stop, compose the shot, get the right settings and squeeze the best out of scene.
Here we have two bikes in Bologna Italy which caught my eye. Maybe it's the interrelationship between the wheels, maybe the colours, I don't know. I stopped took a few photos, although I should have taken longer, but at least I stopped as I still like this photo.
So keep looking out for images and if you like it, shoot it!