PS
Maulkin, you are actually pretty close to it when you say "over correcting".
In a steady heading sideslip you are using one control (aileron or rudder) to command a turn on one direction then using the other control (rudder or aileron) to command a turn in the opposite direction cancelling out the first command.
Effectively "over controlling" one command with the other to cancel them out.
Andy P; in straight and level flight you are not yawing by definition (yaw being an angular motion how can you be yawing in straight (ie constant heading) flight?). You may or may not be sideslipping (if the balance ball is not in the centre you are sideslipping) but will still be flying straight (ie constant heading).
Of course we don't want to fly like that (it's inefficient due to increased drag and it's uncomfortable) so we would use rudder to yaw the aeroplane (angular change) aligning it with the relative airflow and centring the slip ball. We may or may not then have to hold some rudder to maintain the ball centred; if that's the case we need to adjust the rudder trim so that the aeroplane maintains straight flight, in balance and with no requirement for the pilot to hold a constant rudder force.
Happy landings
3 Point