Once the dog will obey the sit command for a reward with no help from your body language, you can teach the dog that holding the position for a longer period can produce more praise and rewards.
Still begin the exercise by saying the dog's name and giving the "sit" command with no body language. Now, instead of praising one time with a single reward, you give more praise and treats at a rapid interval if the dog remains in position. Giving the rewards rapidly is especially good for excitable dogs that will not stay still for a second. If the dog pops up from position it is OK as long as you do not release the treat until the dog settles back into the sit position. The idea is that there will be a treat AVAILABLE to the dog at a quick interval if the dog happens to be in the last commanded position. If the dog starts to settle, you can slowly work to spacing the reward interval to several seconds. But, at the very least you want to work the most excitable dogs to hold the sit for at least a measly 2 seconds before another reward is given. When the dog seems to understand that holding the command for longer than a moment will produce more rewards, you can move to the next step, DURATION.
Since this is a rather easy concept for the motivated dog to grasp, the next step can usually be started within the same training session!