I use a screwdriver that is as wide as the slot and I make sure it goes ALL the way to to the bottom of the slot
A little tapping might help. With the screw driver in the slot GENTLY tap the handle end with a hammer.
(This idea of this is to vibrate loose the rust/grime)
Liquid Wrench (easy to find) or Kroil (better but harder to find.)
PB blaster works(from autoparts stores), but I've been told to be careful not to get it on the rest of the machine
A little heat from a hair dryer also helps.