Start by yourself with both fists on the wall or the ground and slowly go down from the elbow of one arm and come back and repeat the same with the other arm. Do this a few times  using different alignment of the fist and distance and hight in regard to your body and get to know the different levels of comfort within each movement for you. This will allow you to better your fist alignment from the elbow and be able to transfer more whip without resorting to tension.

Now with a partner repeat the drill with him pushing you in various locations toward the wall/ground and again note how things change with a little pressure. He can also simply rest on you and let the mass do the work. Also different points of support will create different patterns of aligning tension and you are better served knowing how to deal with more.

now have the partner cup your fists in his palms and either he rests his back on the wall or lies on the ground and you both do the drill one at a time or later on together (avoid the face) and again learn to use the best alignment and from this know where to hit and how to place yourself.

Smile or just relax your face. Avoid focusing completely on one point you will not gain much this way. Spread your awareness through the body.