Excellent questions, I'll have to add / expand on that stuff later in the guide. Here are some quick answers below...
Is there a quick way to break them in?
Yes... but you have to know what you are doing to do it as it could easily damage the grips
The wrist area I fold over back and forth many times both horizontally and vertically as well as forward and backwards.
THIS PART IS NOT RECOMMENDED FROM THE GRIP COMPANIES (DO THIS AT YOUR OWN RISK):
The actual leather that goes over the palm, I twist it like I am wringing out a rag. Be very careful doing this as you could tear either the threads or the actual leather of the grip. When you twist them like this you can actually feel the leather stretching. Depending on the stiffness / softness of the grip I either do it only one direction, or I follow up the first twist with a second in the opposite direction.
The next thing I do is have the athlete put on the grips. Once the grips are on I physically put their hand over the bar cable exactly like I want it (with the grips grabbing the cable correctly). I have them pull pretty hard several times to get the grip like I want it.
Now I have them grab the bar and I visually inspect the grip as it is grabbing onto the bar. I look at how the grips fit, how the athlete is holding the bar, how they are wearing their wristbands, how tight the grips are, and if they have the grips in the center of their palm.
From their I have them do some spotted swings on the low bar with bent legs if they fit. I spot by holding the wrist area in case they peel off. If that goes well, then we move onto spotted kips. I visually inspect the grips as they are going while being ready to catch them in case they peel.
If everything is going well and the gymnast can do giants, then it's straight to the pit bar for several turns of giants. If they can't do giants, then also the pit bar, but tap swings instead.
Typically it takes our athletes about 15 minutes to break in a new pair of grips if they already have grips. About a practice if they have never had grips before.
One thing to keep in mind is that if they have never had grips before you may not be dealing with breaking in the grips. You might actually be dealing with an athlete that either doesn't understand how to use them or they are just not ready for them yet.
They always seem to stretch with use making it tough to know what size to order next. Is there a solid reliable sizing method you like?
This question is much harder to answer than the last one as there is an extremely reliable method, but you won't be able to do it online.
Yes... I use the sizing chart from the company along with visually inspecting the athlete grabbing the bar with them on and using them on the bar. We have all sizes of grips in stock at our club, this allows us (coaches) to visually / physically size our athletes on the spot, this is what I would recommend.
Many times I do give them the size that their current grips have stretched into.
The thing to understand about sizing grips is that a professional coach should always approve them before the athlete does giants or any 360 degree circling skill in them.
The biggest thing to remember is that it is much better to have to pay for two pair of grips because you ordered the wrong size the first time, than it is to pay for an injury.