I think, most importantly, for it to be training vice practice it must have a predetermined goal. I try to establish up front what I plan to accomplish. It's pretty easy for me to get frustrated (practice 25 yard head shots, for example) and just start wasting ammo. One thing that works well for me when working on accuracy is 5 rounds per magazine. It slows down my rate of fire, makes me concentrate on fundamentals.
My next purchase will be a shot timer. Add another dimension to measurement of skills, and improvement.
Not very specific, sorry. But this works for me regardless of the reason for the trip to the range.