Start with compressing the stock sear spring. It is easy to do if you put the spring over something like a screwdriver and use a washer to compress the spring. Do this slowly and test periodicially in the pistol. Some folks end up snipping a coil or two off the spring to further lighten the trigger pull. As Crosshairs mentioned, you can order the parts for an adjustable spring mechanism from Crosman, too. Another option is to order a trigger spring kit from TKO. That is a drop in, noticable improvement.
Always do a bump test to insure the pistol does not fire accidentally. You can reduce sear spring force too much and the sear can disengage the hammer when the gun is bumped or dropped.