Fill up your charge with normal medigun, stay inside your spawn point, uber, while ubered quickly open game menu and go to character weapon setup. Choose medic and choose to change to keep any weapon or change between any weapon but the Blutsauger or Syringe Gun before uber wears off.
When you are back in the game you will notice you will be wielding your syringe-gun/blutsauger and your uber will last forever. You can then change to any weapon and it'll stay. But if you change to Medigun and then back to a different weapon it'll wear off.
If you changed to Medigun and want to keep self-uber go back to spawn and choose to change to keep any weapon or change between any weapon but the Blutsauger or Syringe Gun.
It wears off if a Medic or dispenser heals you.
To infinite kritzkrieg yourself.
Do exactly the same thing expect with Kritzkrieg this time. Self-kritting only affects Syringe-gun and Blutsauger. All your Syringe/Blutsauger shots will be crits, yes even the Blutsauger will crit.