If you're talking about the species, there is only 1 way known to be both reliable, practical and succesful: through the Hogyoku. There are 2 ohter methods, but they don't possess any of the 3 aformentioned advantages: Aizen's Hollowfication Virus, which, if left untreated, turns it's victims into a full Hollow; and being born from someone infected with the aformentioned virus, but who was stabilized.