I have no clue what
@TypeRx is doing to get another 2 FEET of stretch from a 20' half.
I got like 1 or 2 inches max. I just looped it around a curtain rod and leaned all my body weight into it for a minute or two.
I barely get any added length, but do feel the string pulling like a rubber band. It removes some coil, and that's good enough.
Many here are able to get ~20-24" out of a 20' run during pre-stretching, so it isn't anything unique to me. Make sure you are tying one end to a fixed object (as is, much stronger than a curtain rod!) and then use your preferred method (I use a baseball bat; string tied in a knot around the 12" from the bottom of the bat with bat head on the ground) and then lever it back. There is something seriously wrong if you are only getting 1-2". Honestly, that is what you get just from removing the coil. It takes just a few seconds to get the first 4-6" of stretch.
Some use a doorknob (I tried that and wouldn't recommend it),
@graycrait uses a winch, others similar to what I do. I find it better/safer to do outside rather than inside. You also need at least 30' of runway since you have 20' of string, a couple feet of stretch, and some space to maneuver. Don't try and overlap the string and pull on 2 ends at once -- like
@travlerajm said that doesn't work (essentially halves the actual pulling force) and creates another bend for the string to weaken and break on when stringing. I tie knots around the fixed point and the bat -- please don't just loop the string around something. Not only will that not be as effective it is also more dangerous.
Regarding string breaks when doing the mains, I have found that natural tends to break more during stringing then black and also plays worse than black. I have "successfully" broke ZXP mains while stringing after pre-stretching, so I am not sure if the pre-stretch helps with this aspect. It is a picky string, and this is why I don't use it as a main any longer.
On the sets of ZX that have broken -- don't toss them. Save them for when you choose to string the crosses using 2 pieces or pack them up and mail to me!
Maybe I should take a video or some pictures of how I pre-stretch this stuff. I don't really need to restring anytime soon (maybe after another 20 hours of play of so...) but can do it next time I string up if there is interest.