I think I know *why* a lot of people lose badly.
Firstly because they build in errors by working on past mistakes in training, and by paying attention to mistakes.
Secondly because they don't start with enthusiasm.
Thirdly because a show is a very different environment to where a lot of people train, and where owners act weird.
And, fourthly because they practice whole exercises.
The first one is easily fixed. If your dog looks away when teaching
Do small amounts of heelwork perfectly, not large amounts with faults in the middle.
If your dog doesn't do a perfect front, don't give the cue to heel. Chains are reinforced by giving the cue to continue the chain. Don't reinforce mistakes by cueing the next part.
Never, ever pay attention to mistakes either. This means don't reinforce a mistake by trying to get your dogs attention. The classic one most dogs learn is that if they look away or wander off while heeling their dad gets liver out. Over time I have watched people train their dogs to heights of innattention with this method, and I have done it too.
Secondly train with play. Throw food so your dog has to run and get it, play with toys, play with your dog without toys, surprise your dog, use FANTASTIC treats, use rubbish treats, use bizarre treats, but get them off your body as soon as possible!
Thirdly train in weird places, train all over, train in new places and allow your dog to explore them after he works. Of course if he doesn't work, he doesn't get to go explore.
Train where you feel embarrassed. I find any where public embarrassing, but places with bigger audiences worse, like lawns outside schools, high streets, public parks.
And, fourthly don't practice whole chains. Split 'em up. A typical practice session for me would include my dog sitting at heel while I throw toys and food out, heelwork through all those distractions, sending my dog to collect a distraction, calling it back to present, sending it for a toy, calling it back through my legs, taking the toy (no present) getting a finish then leaving in a stay, walking away and turning suddenly waving my arms in the air etc, then releasing the dog to go fetch another toy, and so on.