Practice :) For smashes, being loose until the last moment and shortly before impact with the ball build tension. The movement is more natural, makes the ball jump higher and is better for your arm. For other overheads, bandeja, vibora, bajada, I suggest getting a coach, who can show you the proper technic. From youtube videos it is very hard to see and feel exactly what the right technic is. And above all practice and experience :)I hope thid makes sense :)