This has become a bit of a bugbear of mine - having seen various horses prove fractious in the stalls, get loose at the start, miss the break, lunge left or right - mainly 2yos but in the wake of seeing the US horses at Ascot and how quickly out of the stalls they were in relation to our UK 2yos but:
How much stalls training do British trainers actually give their 2yos?
It can't be too much to ask that your horse is more than aware of what it needs to do at the start of a race.
IS, generally a trainer will put a horse through 3 or 4 times prior to a race. At Channon's, whilst they are yearlings in December all the yearlings one morning will walk through them, then stand in them, prior to stand in and it open and just walk out. The first lesson is to mainly educate them in going in and not to be scared by them, giving the horses confidence.
The around mid February time (again in the indoor ride) all the 2yo's will go through 3 - 4 times one morning. First time walk through, then stand in and let them jump out at their own accord. The next two times, you pop them out, but baring in mind you only have around 50 yards to a bend, you just let them hack away.
The ones who are good here won't have any more training but around 50 - 60 who aren't natural will do it once more a few weeks later.
Normally a day or two before the horses debut, they will be taken up the gallops and placed in a three bay stall with normally another 2yo about to run and an elder horse. All three are lead in and popped out, this would be their main stalls work. They would do this again 3 or 4 times, until you are happy that they can jump out into an adequate stride.
Also with all the yearlings at Channon's. Throughout the winter after they have done their 5 or 6 3f bunch canters, you walk home into the picking paddock. Prior to picking on their backs there is a man made wooden stall. To get into the paddock you need to walk through this. Each yearling will walk through on the way back into the yard, and will do this every day for a good few months. Some will jib or need encouraging, and some refuse point blank. But they all seem to get used to it and its good education for them.
I would expect this would be a similar process for most leading yards.