Balance: Balance shifts a bit during the service motion. Turn away and toss straight up with more weight at first on the front foot during the tossing phase (or at least evenly balanced. Don't toss off the back foot: it's inconsistent) As you release the ball, and begin to coil or lean back, shift your weight back onto the back foot (back-loading) Keep elbow away from body and up.
Leg drive is important as well. Even though, there's been a bit of controversy on this topic with some teaching pros challenging the importance of leg drive. Leg drive seems to put significant kinetic energy into the service motion and allows for a one, two, whip-like motion with the leg drive, as part one, and then the trunk and shoulder twist followed by the arm and wrist snap as the second part or final whip.
Leg drive video example 1 Federer