Well, the first rule of handling is to buy the best R-compound tires you can.
Increase your front tire pressures and reduce rear pressures (in 2.5 increments) if you want more oversteer and less understeer.
Likewise, if you increase the width of the front wheels while decreasing the rear, you also increase oversteer.
If you buy new dampers, decrease the damping forces for the fronts and increase the rears.
Other things you can do is get springs with different spring rates for the fronts and the rears (soft for fronts, stiff for rears). Suspension bushings also can help (stiff fronts, soft rears) and like Ward said, a sway-bars help as well (soft fronts, stiff rears).
Of course, use common sense and if you try to do all of this in one install, you may eventually get excessive oversteer.
EDIT: Anything opposite would conform to the opposite (e.g. increase understeer while decreasing oversteer).
I'm sorry about my outlandish behavior - there's too much blood in my alcohol system.