The force required to turn a screwdriver can be reduced by ?

The force required to turn a screwdriver can be reduced
by making the handle of the screwdriver fatter.  

The torque applied to turning the screw is (the force you apply
to the outside of the handle) multiplied by (the radius of the handle,
away from the central shaft).

By making the radius of the handle larger, you increase the torque
applied to turn the screw.