For me I just started to add the command as we are walking since Hector is pretty responsive to my voice. I would say the command (he didn't know what it meant at first), the moment he stopped, I would mark and run up to him and treat. Start this with the dog beside you and walk and say your cue. If the dog stops momentarily to look at you, mark and reward. Do this a few times with the dog next to you. Then try it with a bit more leash length. Do this when the dog is pretty calm and not distracted by too much. First call their name and if you see they are paying attention, then add the stop command. Work it up from there (long lines, off leash, dropped leash, etc.)
There is also this game called "go wild and freeze". A trainer I went to in my area came up with the idea. Here's a video demo. I would not wait for the dog to sit. I would mark and reward for the dog stopping and giving me attention. When on leash, my requirement is for the dog to stop or stop moving forward.
[video=youtube;TzM0nvlQPsY]https://www.youtube.com/watch?v=TzM0nvlQPsY[/video]
Good luck.
