The basic cross is mainly used as our power punch and it requires a full body commitment to maximize its effectiveness.
What's great about martial arts is that you don't have to be super strong or fast, but to be able to apply such technique correctly at the moments necessary.
Energy (power) starts from the ground, to your feet, to hips, shoulder, and finally your arm as the follow through. As you throw the rear punch it's important that you transfer the weight from the rear to your lead leg.
Exaggerate the pivot (point rear foot forward), rotate your rear hips forward, turn rear shoulder forward (as you do that lead shoulder goes back)
Arms is relax for effective movement then tighten the fist upon impact. Make sure your wrist is straight or you will be hurting yourself.
As for every punch, the fundamentals apply: chin is down, eyes forward, elbows in, breath out, athletic stance, neutral spine (not leaning forward or leaning back). Hit the intended target and bring your hands back from point B to A.
If you are out of range, have good habit of closing the distance by using your feet to step in rather than leaning forward (compromising your structure).
Have confidence in your strike and understand that your body (mainly your legs) plays a big role in its effectiveness, and even more so when you're tired. Remember to commit with your whole body and not just your forearms.