Stretch!!!
It still surprises me how much older skaters complain about pain but refuse to give stretching a try. It does wonders. Foam roller is also key. I typically stretch around 30 min before a session usually with some coffee, i feel it helps warm up my body along with the stretches.
I recently relearned kickflips from hearing Jeremy Wray's explanation of it. Criss crossing the pressure from feet to feet before you pop to creates a strong centered pop. If that makes any sense.
The more you hang your back toe on a 360 flip the easier it will pop.
You CANT be light footed on a hardflip, its a one of the tricks i feel i create the most pressure before popping out of any flip trick i can do. The more you create a centered pressure towards the back of your board the faster and easier it will flip. The flick is minimal.
When i do a nice leveled FS pop i feel I keep my upper body slightly back, for some reason i feel that helps me stay leveled on that trick than staying completely centered or leaning forward.