Just my experience...
Range time.
Mindful dry fire -- learning the characteristics of aim and trigger control on this specific model.
A gentle smoothing of the contact points in the lockwork narrowed my already decent groupings noticeably. This should be done only after you've learned the proper technique, or by a qualified S&W revolversmith.
Leave all springs alone.
More range time. Should be at least a couple hundred rounds down range for familiarity and to determine reliability.
Alessi Deep Cover is a superb inside the waistband holster designed specifically for J-frames; don't expect them to deliver it on time, but they will deliver it and it's worth it. Alternately, Lobo Gunleather's Offset Belt Clip in horsehide is a close second.
Skip pocket carry.
Carry the 642 unloaded around the house to acquaint yourself to the feel, and practice unloaded drawing, aiming and firing drills adhering strictly to the four cardinal rules of safe firearm handling.
Once you're acclimated to carrying, drawing, and shooting (range style) a small snubby, seek as much as you can comfortably afford of one or all of these rounds:
1) Speer Gold Dot Short Barrel 135gr .38 Special +P hollow point
2) CorBon DPX 110gr .38 Special +P hollow point
3) Buffalo Bore 158gr .38 Special Standard Pressure lead semi-wadcutter hollow core
4) Remington HTP 158gr .38 Special +P lead semi-wadcutter hollow point
5) Hornady Critical Defense 110gr .38 Special +P hollow point.
Send these rounds through your 642 at the range to determine the best balance between what your gun and your hand like vis-a-vis accuracy, reliability and comfort.
Select a favorite, buy more of it.
Carry.
And consider investing in a good self defense focused shooter's course.