One exercise you can try is to draw something from refs (life or otherwise,) then try drawing it from imagination, then check the refs to see where you're off, then try again, keeping your mistakes in mind and trying to correct them... repeat until you get it right.
And what Jeff said, times 1000. If you want a certain level of realism, you're always going to need some sort of reference. Almost nobody does realism without reference; usually people who work without reference have a somewhat stylized approach.
Professionals use reference. Amateurs don't use reference, and then brag about not using reference.