According to Dr. Nicholas Dodman, once you have a dog performing the desired response greater than 85 percent of the time in a quiet undisturbed environment, you can move onto the next stage; starting to shape the behavior toward the ideal response. You might begin by rewarding a progressively faster SIT, that is, rewarding the dog for sitting in 3 seconds, later in 2 seconds, and ultimately in 1 second, or immediately.
Decide before you give the command what you are going to reward. You can also start to reward longer and more definite SITS so the dog has to do more than just touch his rear end on the ground to receive reward. Withhold the food treat until the dog is sitting properly and then gradually introduce a time delay before the reward is given.
Dr. Dodman has some great tips in this article – please read 12 General Rules for Training Dogs.