Question

various  techniques  used  by  human beings  to  reduce friction

Answer 1

Use of ball bearings
Use of air cushions
Oiling or greasing
Using anti-frictional alloys 
Polishing
Streamlining objects
:)

Answer 2

Some basic techniques used to reduce friction:
1.Polishing :
  If surfaces are polished irregularities are removed and there by friction   decreases.
2. Use of ball bearings:
  Ball bearings convert the sliding friction into rolling friction.Hence, the friction is     reduced.
3. Use of Lubricants:
  Lubricants fill the irregularities of the surface and there by the friction is reduced.
4. Stream lining :
  Stream lining is done to reduce friction due to air.