How to Recover from a Spin in Cessna 172?
The memory aid is PARE:


POWER to idle
AILERONS neutral
RUDDER on the floor opposite the direction of spin
ELEVATOR nose-down to break the stall


You will want to read the full explanation in the Pilots Operating Handbook or POH for an understanding of the steps and why you need to take them. Commit the sequence above to memory and better yet get spin training from a qualified instructor.

The POH for the 172N model reads (caps and boldface in original):


  Should an inadvertent spin occur, the following recovery procedure should be used:
  
  
  RETARD THROTTLE TO IDLE POSITION.
  PLACE AILERONS IN NEUTRAL POSITION.
  APPLY AND HOLD FULL RUDDER OPPOSITE TO THE DIRECTION OF ROTATION.
  JUST AFTER THE RUDDER REACHES THE STOP, MOVE THE CONTROL WHEEL BRISKLY FORWARD FAR ENOUGH TO BREAK THE STALL. Full down elevator may be required at aft center of gravity loadings to assure optimum recoveries.
  HOLD THESE CONTROL INPUTS UNTIL ROTATION STOPS. Premature relaxation of the control inputs may extend the recovery.
  AS ROTATION STOPS, NEUTRALIZE RUDDER, AND MAKE A SMOOTH RECOVERY FROM THE RESULTING DIVE.
  
  
  
    NOTE
    
    If disorientation precludes a visual determination of the direction of rotation, the symbolic airplane in the turn coordinator may be referred to for this information.
  




Other Spin Recovery Methods

The methods below may or may not recover from all spins. Seek out a qualified instructor for spin training — or, better yet, avoid spins altogether!

Finagin


  Finagin advocates a four-step recovery technique for unintentional spins and departures from controlled flight that pilots can put to use without hesitation — and without fear of making the situation worse — even when they’re confused or disoriented by the unusual attitudes and strange sensations that can accompany spins.
  
  
  Rip the power to idle. 
  Forcefully neutralize the stick and rudder pedals. 
  Look at the airspeed indicator, and once it shows 100 knots or more (roughly 1.4 times the airplane’s stall speed) … 
  Pull out of the dive.
  


Source: AOPA

Beggs-Mueller


  
  Power – Off.
  Remove your hand from the stick.
  Apply full opposite rudder until rotation stops.
  Neutralize rudder and recover to level flight.
  


Source: Spins in the Pitts Special by Gene Beggs

Hands In Your Lap

Some advocate the “hands in your lap” method of pulling power, taking your hands (and feet) off the flight controls, and waiting for the airplane to fly itself out of the spin. This may work for certain airframes but not all. Catherine Cavagnaro of Ace Aerobatic School says she likes to perform spin training in a Cessna&nbsp;152 because that airplane will not self-recover from a spin.