Drill, drill, drill.

Seriously.

There's no trick to it. There's no technique to it, secret or otherwise. They just have to do it, over and over until they can get it.

I spent many an hour as a young lad in a band playing King marches where the horns, tubas, bari saxes, and bass clarinets (and usually 3rd and 4th cornets too) had to play it, over and over and over until they got it consistently right.