TWO-HANDED BACKHAND: (for lefty) Make sure your right leg is continuing out after contact with the ball, so that when you are finished your swing you are standing square to the net. If you leave the right foot behind, this will eventually be taken advantage of by your opponent, because they will move you further and further out of position.
TOSS: Make sure your toss is not going too far left. You want the forearm to move slightly more to center.
BACK FOOT: Be sure that your back foot kicks out as your anchor. Most professionals will have their back foot all the way above their head to create more spin and power.
RIGHT FOOT: (reverse for righty) Let the foot land first. You don't want both feet to land at the same time. The back foot is anchoring, while the right foot is thrusting forward. If you see a player landing on both feet, you can take advantage of them, since they are less prone to be in position once the ball is returned.
KICK: Put the ball a little behind you. The ball will leave your sight line for a moment. That's how you'll know if you've tossed it correctly. Then push up through the ball, brushing up on it as you would for a forehand, or any other shot where you're creating topspin - except you're doing this in the air, and in a different direction.
SLICE: Toss the ball in front and a little left. Be sure to swing around and down through the ball.
FLAT: Toss the ball more out in front. Be sure to have a full extension.
SLICE/KICK: You can add a bit of spin on the slice to make the ball curve a bit. Just hit up and through the ball, and make sure that your trigger finger is brushing up against the ball slightly.
TRIGGER FINGER: The trigger finger is the index finger that curls up on the racket above your grip. A server can can change the style and impact of spin by where they apply pressure with the trigger finger. Players need to make sure that the trigger finger is extended out, but not too much. You don't want the pointer finger completely flat on the grip while the rest of your hand is curled around; it needs to be out and slightly curled under - like a fish hook!
That was what I learned today. I have another lesson on Sunday morning. : )