I've taught Honey to drop on recall and I also randomly "drop" her when we are out on walks and she is just trotting around, sometimes away from me. She has a very good instant drop for a giant breed.
Definitely get the dog used to following commands at a distance.
Then you can also introduce commands while they are moving - eg, for drop on recall, I call her and run backwards with her running in front of me - then I give her the drop command (we use "Mat!" as opposed to "Down" - slightly different arrangement of the legs, allows her to go down and come up faster - things you have to consider when you have a 70kg dog!) and hand signal. I might pause to make sure that she is starting to go down - but then I keep running backwards. So she learns that when I give the command, she has to drop and remain there, even if I'm continuing to run backwards. Once I'm some distance away, I give the release command and lots of praise for her running towards me.
Then we practised drop from a distance. Then we combined the two - I would call her to me and when she was quite close - give her the drop command and huge praise if she even hesitated or slowed down and of course, if she actually goes down. If she is doing this well, I would start giving the drop command earlier and earlier so that she is dropping further and further from me.
AT THE SAME TIME - I was randomly giving her the drop command when we were out on our off=leash walks - so that she understands the command is to drop down at any time, not just only when she is doing recall.
Honey caught on pretty quick. To prevent her anticipating it, I would practise normal Recalls and Drop on Commands together and vary which one I would ask - so she never knew if the next exercise was a straight recall or a drop...so she always comes very quickly. Sometimes she is slow but that's not because she is anticipating the drop - she can be slow on recall too - that is just her being slow because she is a giant breed and she is overheating or getting tired and losing concentration or whatever...
She did go through one period when she was dropping a bit slowly and I solved this problem by throwing my keys at her front feet, AFTER I gave the command and IF she hesitated. I know this is not completely "positive" but I know my own dog and that she has no noise phobia, etc, so it was safe enough to do and it worked like magic. I only had to do it once - and after that, she dropped instantly whenever she heard the Mat command. But of course, I don't recommend this for every dog. This is what worked for me. You could probably achieve the same if you just try to click for faster drops - but that can sometimes take ages, especially if the dog is not offering faster drops for you to click. Sometimes, in my experience, it does help to introduce a "deterrent" to give the dog more of an incentive to make the right choice...but I know that is controversial, especially on a "positive" forum like this one.