First of all, congratulations on already having the basics of a good serve.
But if there is one overlying element to add to your serve it is to have more of a
cartwheel motion.
By cartwheel motion, I mean more shoulder over shoulder action, like you see here in this slow motion video of Soderling serving (going into an incredible archer's bow and then reversing this bow with incredible shoulder over shoulder action. All the pros do this, not just Robin.):
http://www.youtube.com/watch?v=a56pvP1i6x8
I think if you can achieve this, you will solve the problem of falling off to the left that SystemicAnomaly describes above. (You already have the coil/uncoil motion in your serve, but that motion without more cartwheel will have you rotating too much to your left and not driving forward enough.)
It will also help add the accuracy (from increased spin) and power (from the cartwheel itself) you seek, as well as having you be able to aim the serve where you want it to go (aiming is cartwheeling in the direction you want the serve to go).
Djokavichfan4life tells you what you need to do to get that bigger cartwheel or shoulder over shoulder action : "Right now you don't really shove the knees outwards and lean back with the upper body. If you did this you could get a better angle upwards with the shoulders and hit up on the ball more."
But perhaps some additional videos can let you better visualize how to do this:
How to keep bringing your tossing arm up as you coil and start into the archer's bow smoothly all at the same time with great balance:
http://www.youtube.com/watch?v=CeZp90h-Ar8&feature=channel
Good video of multiple pros coiling and going into the archer's bow as they continue to bring their arm up:
http://www.youtube.com/watch?v=lIF-UaRUd6k
Will Hamilton from Fuzzy Yellow Balls emphasizing how letting your front hip protrude into the court will counterbalance the backward upper body lean so you can get a great archer's bow:
http://www.youtube.com/watch?v=HgeYmEScfgQ
A front view of how your serve should look with good shoulder over shoulder cartwheel action (with the dividend that this motion is actually good for your hitting shoulder because it puts less stress on the rotator cuff):
http://www.youtube.com/watch?v=lTRvxaBMh8s&feature=related .
Jim McLennon continues in this next video to also point out how the racquet head rotates across the ball to get that needed topspin action on the ball for a greater first and second serve percentage as you are coming through your cartwheel (I think you already have good pronation action - just don't lose this aspect as you add in the cartwheel.)
http://www.youtube.com/watch?v=1t6bLABbebc&NR=1
Serve direction is just cartwheeling in the direction you want your serve to go:
http://www.youtube.com/watch?v=v9aQuJyccak
Good luck!