First off, does the muzzle fit correctly? I have had to try different muzzle to get the right fit for my dogs.
Once I have a muzzle that fits I then only use it when they are running and by then they have practiced and have a strong desire to chase the bag. For my one that had a bad experience with her muzzle and was having issue we would use food to help build a positive association with the muzzle. Spray cheese is one of the best since the nozzle fits into the basket. For the most part my dogs will ignore the muzzle when they run and then fuss afterwards.
It is good muzzle???
Here I have problem with coursing muzzles… Sightound`s owners muts to buy Pro-coursing muzzles in other countries. And there are rare too...
The dog in the pic you linked to is my girl Rally and she wears a size 4 because anything bigger bumps up into her line of sight and she will not run. She is also the one with a negative association to the muzzle.
Here is a pic of boy Nicky, he wears a different style muzzle that all my other dogs find much more comfortable and more difficult to get off.
Here is a website with a picture of the muzzle not on a dog, scroll down to the bottom. They unfortunately only come in one size so are somtimes a little big on a petite girl.
For racings. If I want licence- he must runs with muzzle. I try to learn him but I won`t to do mistakes…
What I do for training them to wear the muzzle is go to the trials (or races) as we have practice usually after the trial is over… I start putting the muzzle on then for just practice.. and that is usually after I run them without one to see what their level of interest is in running.. if they are keen to run, the muzzle will not make a difference... and they begin to learn that the "muzzle" means "running"... all good things...