To reduce oversteer (increase understeer):-

Reduce weight transfer at rear by reducing rear roll stiffness
Increase weight tranfer on front by increasing front roll stiffness
Reduce aerodynamic downforce on the front tyres
Increase aerodynamic downforce on the rear tyres
Use wider rear tyres
Use narrower front tyres
Use heavier front springs
Use lighter rear springs
Use a heavier front antiroll bar
Use a lighter rear antiroll bar
Move some weight forward
Use softer rear shocks
Use harder front shocks
Use more negative camber at the rear
Use more positive camber at the front
Lower the front tyre pressure
Raise the rear tyre pressure
Make the rear track wider
Make the front track narrower