For me, the best thing that added the most pop on my serve was to throw my elbow. Throw your elbow and let it lead, and your arm will whip right out in front.
The second thing that allowed me to get better was upper body rotation. Uncoil on the ball, don't be in one position. I stand sideways but end up foreward.
http://www.tennisplayer.net/public/...rchive_sample_archive.html?AR1stSAdFront1.mov If you watch, his body goes from 90 to 180 (90 degrees), that is uncoiling and upper body rotation. It allows you to hit the ball much with much more force and speed.
https://www.tennisplayer.net/public/biomechanics/BE_Power_Serve_Part1_sample.html Even more so, watch all the clips they all do it.
The third thing that helped me was a consistent toss. Now, I can hit my old fast serve with a slow swinging motion because I can make near-perfect contact. And now I can make sweet contact with the ball, the ball just flies off my racquet.
Out of those, I think the consistent toss and near-perfect contact helps the most. Because it's that, that makes it all happen. My old toss, old loopy swing, trophy position... All that equated to me hitting a ball maybe 60-70 MPH at most. Now I can hit that with a slower swing and hardly any movement and it's because of contact.