Thanks to Ian Westermann, EssentialTennis.com for this tip on tennis toss consistency. He suggests that the key to a good serve starts with the toss; and the key to a good toss starts with the correct “hinge.”
Think about your service toss… what hinged joint flexes when you toss for the serve: Your fingers? Your wrist? Your elbow? Your shoulder?
According to Ian (and I agree, for what that is worth), you will get the most consistent toss by keeping your hand, wrist, and elbow fairly solid … and just using the flex of your shoulder to propel the ball to the proper height.
For his video on this subject, with more suggestions on practice techniques, visit his site by clicking THIS.
Check yourself out to see what hinge you use.