3 D's - Distance, Duration, Distraction
Whenever you teach a new behavior, get it going right, in many different locations and situations.
When your dog is doing the new behavior reliably, train for it at a distance - it won't be a very useful behavior if your dog will only do it at your side, while on lead.
Increase the duration of the behavior. "Stay" is an unnecessary cue, when you teach your dog to stop and sit, and to "down", "stay" should be implied. Teach your dog to sit and down for gradually longer periods of time, and until you've released him.
Get the new behavior happening under gradually increased, and gradually more tempting distractions.
Train towards the likelihood that the new behavior will happen in every occurrence you need it. Sit, sit quick, sit square, sit until released, sit way over there, sit to whistle, sit to gun fire, sit to flush, sit attentively while we call these ducks, sit while the marks are going out, sit until sent:….etc. etc. etc.
These are all "rules" we intend for behaviors like sit, don't teach your puppy to sit in the kitchen a dozen times, and then be flabbergasted when he isn't "steady" on the marks next week.