I gave Dakota a new phrase to associate with the action of lying down. I would say the words (I use "hit the deck" and a whistle for long distances) and give him the typical hand signal for "down" shortly after saying the words. Dakota eventually began anticipating the down signal after hearing the words, and would start lying down when I said the words instead of waiting for the signal. That's when I eliminated the signal. Once he was reliably giving the correct response to only the words, I began reinforcing only for the speediest of responses. I only gave this command once or twice a day, at completely random times. For the first couple weeks I kept it to mildly distracting situations to ensure success. I gradually built up the amount of time he needed to stay in the down position to be released. After I was confident that he'd perform it under most circumstances, I moved on to moderate/strong distractions.
Reinforcement was a BIG deal. I made sure that every time he responded to the command, he got a full 60+ seconds of enthusiastic praise, treats and/or tug, then he got to do whatever he wanted. He learned that this command is the signal for a "jackpot", and would go down and stay down until I approached him to give him his reward. I always made sure to go
to him to release him, rather than calling him off, that way he won't ever anticipate a recall and break his stay. In several "close call" situations I've released him by giving a recall command, but in practice I never do.
It requires a little bit of upkeep, I still practice the command every few days, usually before I feed him a meal. I'll set him up in a distracting situation and if he downs, he gets fed immediately. Overall it was a really easy command to teach and has been a lifesaver on three different occasions.