Here is how I train my ponies to park, i dont show moderns but i do show classics and one foundation, so the stretch isnt extreme but a bit more then a mini. Just enough to level out the topline/croup just a bit more (atleast thats how i look at it).
I start asking mine to square up the back feet and then i teach them to stay/stand. I use a 6ft+ lead and i back slowly away until im at the end of the lead, if they move i yank the lead and stomp my feet, they eventually learn to stay right there and i do not want them to move.
Then i will come back up to them after this and ask them to reset the hind feet and i then show them a treat and give a small yank on the halter so they feel just a tad pressure around the poll and ask them to step up with their front feet. If at any time they move the hind feet i give them a yank on the lead and back them up. They will learn when i show them he treat and give them a yank its time to step up. Then i can move their front feet around as i need by pushing on the shoulder ext.
This is also when i teach them to work their neck, once they step up and i have them where i want them i give another small yank (as i did when i was working with them in he beginning) to signal to them i want them to stay there and not move or step up, i then move in front of them a bit and as them to work their neck. If they step up any then i give them a small very gentle tap on the chest with my foot, normally i hardly even have to tap them as it just catches them of gaurde lol.
I've found the shetlands are easier to train and get it faster then the minis
The hardest thing is though, once they learn to park they LOVE to park lol. I have a gelding that will walk up in the pasture and park out thinking i have a treat. If im not paying attention in the show ring for a moment he will just keep parking himself out if im looking at another horse or somthing.
Also, its not imo something you have to do DAILY. I havnt even set up my filly or gelding in 5 months and i garentee i can go out there right now and they will park and show off. I spent about 5 days training my yearling filly and then i was done, that was it, she has it and i garentee she still remembers it
. When it gets closer to show season i might set them up once a weekend or so but really, once they get it they got it
.