According to Dr. Nicholas Dodman, gradually increase the length of time the dog must remain in a SIT-STAY until he can remain relaxed in this position for one minute while the owner is at a distance of 5 feet. Continue to increase the time and distance the dog is asked to remain in a SIT-STAY after the dog has been successful at the previous level for 5-10 trials.
For very long SITS, the reward should be given intermittently throughout the SIT, at least during training. The owner should teach a key phrase such as EASY or STEADY to teach the dog to associate relaxation with the exercise. It also is helpful to have a release command, such as FREE or RELEASE, which tells the dog when he has been obeying for the desired period of time.
Dr. Dodman has some great tips in this article – please read 12 General Rules for Training Dogs.