This is something I've found to work well when teaching this- by that I mean, how to get close and punch effectively from that distance.
When in the ring and working the mitts, I work real close to my body. Every move is brought in close, so that the boxer gets an understanding of the footwork necessary to get in range- in fact, what 'in range' means in practical terms- and how to punch with leverage once there. And, of course, how to get out.
In my opinion and experience, you have to make these sessions as close to a 'fight' as you can get them, with movement and defensive moves worked in, and no tap-tap punches allowed. You have to work sharp to be sharp; talk to the fighter as you work, use the time between rounds wisely, and watch his feet.