Cookie could already sit on command when we got her from the breeder, but the obedience school learned us the hand signs to it and now she sits also when I raise a finger and waits till I give the command: walk. Practical when there are many dogs and she doesn't really hear it. Here's how it's done:

Take a dog treat in your right hand between your thumb and middle finger and raise your index finger while you tell your dog to sit. He sits automatically because he wants to look at the treat. Than give the treat. After a while the dog also sits by just seeing the finger, but of course, he should also be able to sit on the spoken command.