I have been working on the mental game recently. For now, that consists of visualization. I have been mentally rehearsing the left jab-straight right (or right cross).
The mental training is interesting. To do it well, I think one has to imagine that all of the senses are participating in the practice. Specifically, it helps to imagine my feet and legs stepping forward with the jab and pivoting with the hips to really turn the body into the right.
Some of the rest of it is obvious: imagining balance, keeping hands up, looking for opportunities to throw more punches after I see how the one-two goes. One thing that isn't quite so obvious is remembering to breathe. That is essential, too, so that I can develop good habits that will carry over into physical training.