NONE AT ALL!!! DO NOT TOUCH THAT SPRING!
AND YES I AM YELLING!
The trigger spring is also your hammer spring. Making it shorter will cause you more problems that you can imagine. That spring controls how hard your hammer hits the firing pin, how fast your bolt opens and several other things. If you want the trigger pull crisper, notice I did not say light, send it to a gunsmith who is completely familier with the M-14 trigger system. DO NOT ATTEMPT TO "FIX" IT without knowing exackly what you are doing. Having a premature out of battery firing puts about 55,000 lbs of preasure in your face.
Scott