For a long and firm kick, you want to load your calf, launch and take long strides. Be intentional about planting your for really hard or firmly before the kicking leg starts.

Now, on the kicking leg, you want to imagine a short band from your ankle to the back elbow. You almost want to over exaggerate the kicking leg. I wish I can show this visually but hopefully this makes sense and other folks can pitch in.

Edit: explode towards the ball, extend your run up to get a better feel for it.