In a large part I think it depends on the person. For example, I am able to literally visualize electric blue energy coming out of my hands and forming a circle... it takes INTENSE concentration (and for some reason I CAN'T visualize green energy or red as easily, but violet is about as simple). I think it depends on a variety of factors... how imaginitive you are, what kind of memory you have (I have a photographic memory... it also aids in MAKING images in your head and holding on to them... someone with an audio memory or one that relies on touch might visualize energy differently).
But then... you are right that it does take lots of practice. When I was really little my coven would often allow me to cast circles, but I was unable to actually visualize energy with my eyes open until I was in middle school. I had to do it with my eyes closed, or draw invisible boundaries in the air.
However, I actually prefer working with "tools"... not always literal athames and stuff, but physical representations of these things. Back in the days when I DIDN'T have an athame or chalice (and was distraught because I felt I was missing out on something) I was taught by my cousin that the hand alone can be used to represent the basic tools-- the index finger can represent an athame or wand, the cuped hand can represent the chalice, the flat hand can represent the pentacle, etc. I have actually used and seen these used in the circle instead of tools on occassions where either NOBODY had an athame, or when I was practicing solitary and didn't have tools of my own...
I guess it just depends on individual preference...