Hopefully you will find the following video helpful:
Coach McCraw has some nice shots from the back showing correct path of the arm/hand/racquet in bringing the racquet to a full racquet drop.
He emphasizes taking the racqeut back and up in an "outside to inside" path.
By bringing the racquet back away from the body ("outside"), it is easier for it to end up on the right side of your body, or "inside".
[Conversely if you bring your racquet up very close to your body, there is a tendency to end up with your hand/racquet more in the middle of your back, or "outside". This would tend to give you racquet drop into the middle of your back - it subsequently is very difficult to pronate if you brought your racquet up from the middle of your back.]
(Later on when you swing upward out of your racquet drop you will want to pronate.
Many don't realize that the racquet/arm at full racquet drop has to be well to the right to set up pronation in the actual swing phase - they mistakenly have the racquet drop in the middle of their back in a "backscratch" position.)
In the above sequence you can see how Sam Stosur's racquet drop is well to the right of her body in pic 1. Notice how the arm is straightened in pics 1-10 keeping the arm well to the right side - this sets up the ability to swing the racquet from right to left to contact in pics 11-19.) )
Once you have the proper racquet drop well to the right of your body, you can then practice this pronation movement as a separate exercise, as demonstrated in this video:
McCraw explaining a serve pronation exercise http://www.youtube.com/watch?v=iONY6fcqZGg
While the above can be worked on as separate shadowing/hitting exercises that can then be incorporated into your serve, you also need to work on how you get your serve started - your "toss into your trophy position move".
[If you don't get off to a proper start, the toss seems rushed, and it is struggle to maintain balance - rather you should smoothly be in balance throughout the toss into the trophy position.]
Tossing and getting into your trophy position must be combined - you should not be tossing - then getting into a trophy position.
And the place to start is using the forward bend into the court to start to build momentum that you can use to get the toss started. (Right now after you step forward, you totally stop before tossing.)
Notice Fed bends forward (pic 1) , and that as he bends his upper body back (pic 2), that automatically gets the arm starting to move up, starting the tossing motion.
He brings his tossing arm straight up in pics 3-4. Having that arm straight up in pic 4 - the trophy position insures he has a very steep shoulder angle - his left shoulder is straight up, and his right shoulder is straight down.
Notice in his trophy position he is in the shape of a bow - that bow is even more accentuated in pic 5.
So getting off to a proper start in a serve is key to having a relaxed balance serve.
Check out the following three videos on how to get that toss into the trophy position:
Tennis Serve Tossing Motion Tempo [uUd6k&feature=related[/url]
Tennis Serve Toss - How to Hold the Ball http://www.youtube.com/watch?v=M8kyn...eature=related
(It is much more comfortable to toss the ball in this way, rather than trying to contort the tossing arm to maintain the hand under the ball as you bring it higher before ball release.)
To top it all off, dropping the left shoulder is protective of your rotator cuff muscles, and can help prevent a shoulder overuse injury as you are doing all that serve practice!
Preventing Rotator Cuff Injury http://www.youtube.com/watch?v=lTRvxaBMh8s
ope I've given you some tips on how to work on things individually.
I hope this helps.