I get this question all the time - just like Matt Antonelli does (2b for the San Diego Padres). I made a bunch of videos last night (you can check them out at our YouTube channel page), including a video response to this question. While there's not "one thing" that is going to make you throw 90 MPH, there's a common mechanical flaw that I talk about in the video that might help you add a few ticks to your fastball's velocity.

https://www.youtube.com/watch?v=nmZvxNuHTHM&ab_channel=DrivelineBaseball