How can I make sauerkraut to best preserve the probiotic elements?
The ingredients of sauerkraut are very basic--its basically just cabbage and salt (the water is drawn out of the cabbage).  Given this, you will produce the most nutritious kraut using high-quality cabbage and salt with natural minerals.  A high quality sea salt will contain additional minerals that processed kosher and table salts lack (also, it is typically advised not to use salt with iodine--as it apparently prevents the growth of the desired bacteria).  Additional ingredients such as spices will add their relative nutritional value to the mix.  

Obviously this doesn't necessary maximize or preserve any "probiotic" aspect in particular, but as commenters have mentioned these claims are dubious as best.

Also, by the way, you only need to add water if the cabbage fails to produce enough brine to cover it after being salted.  If you do add water you'll want to add additional salt so as to preserve the salinity--typically I've used 1Tbsp of salt per quart of water.