Steel Piston Sear
Steel Trigger Sear
Steel Spring Guide
High Pressure Piston
M150 Spring (usually)
TM, Firefly or Nineball hop bucking
DangerWerx Type B Hope Lever or perform TDC Mod
Use 0.36g or heavier BBs.
After this you could tune more by....
Tightbore barrel of 6.03 or 6.04mm
Steel Cylinder or Teflon Cylinder
Steel Spring Guide Stopper
Barrel Spacers or make your own out of electrical tape.
If you buy a Zero Trigger of some kind (PDI, Laylax or ASPUK) you don't need to but the piston or sears as these normally come with the triggers as they are 90 degree which makes cocking the rifle much smoother and takes less wear on the internal parts such as cylinder and sears.