Everything you do should be a defensive drill. For instance, you never just hit the heavy bag. You slip a jab and counter with a jab. Everything should start with a defensive move- to create an opening to punch through- and end with a move to get you back out of range so so don't get hit.

You can run through all the left hand counters to a left lead, for example. Slip outside and jab, or hook to the body and so on. It is a ton more productive to train the counter with the slip/parry/etc...