If you breed your babies you can teach them from a very early age - we start them at about 3 weeks old to stand 4-square on a non slip surface, then progress to balancing on our knees whilst we are seated through to free stand before they get fed. Food is such a great motivator for cockers!
The adults have David doing a regular practise, even when they are in the park free running, we pop them onto a show lead, do a triangle and a 'stack' then praise and reward.
I go to weekly ringcraft training as well, but the cockers are the only 'hard stacking' dogs there - that is, the other breeds walk up to a free stand and the lead is held above the head and sometimes not even a tail is held , so it's hard to learn if no other 'stacked' breed is there.
What I did pick up from ringcraft is a set of 'show stackers' an ingenous device with four pillars with each pillar only as broad as a cockers foot. Each pillar is on a movebale track (that is an X of 2 pieces of metal) so this can be set up for each dog. As each dog is off the ground by a few inches and sometimes on the table - using the 'show stackers' requires 2 people and the dog must be held, but it's good for training those that 'post' or 'A frame' or fidget. 'Show stackers are available in Australia from Chinaroad Lowchens - there might be something similar in the UK.
It's also useful getting your dog used to having it's head/ mouth held whilst not stacking the rest of them, so they get the idea of you controlling their head. Tjhere are a few ways of holding heads, including fingers in mouth, finger across the nose etc so watch at the show ring and ask people, as well as see what suits your dog - I have one girl who does back flips if I put the finger inside the flew/ lips !
The mirror is great - I also use it for final grooming and shaping of their coat
