A little slip = fastest. But hard in real life due to finding the limit vs finding a wall, and constantly changing conditions - in LFS we get to repeat the same thing over and over so we can push on much further.
Simplest way to do it in a RWD car is just to downshift slightly earlier than normal - assuming you do good cadance braking, the rear wheels will slightly lose traction, making the car a little more eager to turn in. Note the words "slightly"...
In a FWD car it's a little harder - I personally like "loose" setups (i.e. ones which don't have optimal grip at the rear of the car) as it means I can throw the car into the corner earlier, giving me slight oversteer rather than understeer.