G'day Michiel
IMO, there is two things to consider.
One is toughening the skin to prevent blisters and the second is toughening up the muscles to prevent the deep bruising.
There has already been some good advice about toughening up the skin (brine, urine, vinegar, alcohol, etc).
Regarding toughening up the muscles, IMO there is nothing that will take the place of using your hands for hard work.
A good analogy is pre-season training for Rugby.
You can be aerobically fit, but not match fit (ie able to take the impacts that comes from a contact sport). The only way to develop the match fitness is by taking the impacts that come from playing the game.
If you can use you knives daily in the activities that make your hands sore, push through the discomfort and within a month you wont notice it any more.
Works for me
Kind regards
Mick