What are the symptoms of vortex ring state?
Vortex ring state is characterized by rapid descent and reduced effectiveness of control inputs. At the onset, it is characterized by increased vibration and buffet of the airframe, with uncommanded changes in helicopter attitude. Also, any increase in the collective is ineffective and counterproductive. 

From skybrary.aero:


  
  Incipient vortex ring conditions are typically:
  
  
  increased vibration and buffet,
  the onset of small amplitude ‘twitches’ in pitch and evidence of longitudinal, lateral and directional instability.
  
  Established vortex ring conditions are characterised by:
  
  
  a very rapid increase in rate of descent towards and beyond 3000fpm,
  reduced effectiveness of cyclic inputs in roll or pitch
  the application of collective pitch failing to arrest the rate of descent and usually increasing it.
  
  


From FAA Helicopter Flying Handbook- Helicopter Emergencies and Hazards:


  A fully developed vortex ring state is characterized by an unstable condition in which the helicopter experiences uncommanded pitch and roll oscillations, has little or no collective authority, and achieves a descent rate that may approach 6,000 feet per minute (fpm) if allowed to develop.


The best measure against vortex ring state is quite simple- go forward i.e. increase airspeed. Basically, you enter into forward flight so that the rate of descent doesn't oppose the induced flow. You need 'undisturbed' airflow over the rotor disc. Power is to be applied after gaining sufficient airspeed. From skybrary:


  
  Incipient Stage ... Keep the collective position unchanged and apply forward cyclic to achieve an accelerative (nose down) attitude so as to increase forward airspeed quickly. As soon as a steady increase in airspeed is indicated, and above 30 KIAS, more power can be applied if necessary without waiting until the best rate of climb speed is reached. 
  
  If this action does not resolve the situation rapidly then it is best to treat the condition as established and take the actions below.
  Established Condition. Recovery can only be effected by changing the airflow around the rotor and will inevitably lead to significant loss of height, which makes recovery from a low level occurrence impossible.
  
  There are two theoretically possible actions: Moving the cyclic forward and lowering the collective. Combining these actions is likely to produce the quickest recovery with the least height loss. 
  
  Application of forward cyclic should increase airspeed but a large input held for several seconds may be required before significant pitch attitude and consequent speed change is achieved, with a significant nose down attitude resulting. Lowering the collective to reduce power towards auto-rotation, so unstalling some of the inboard portion of the blades, may also be effective but forward airspeed must be gained before power is re-applied during recovery.
  


Autorotation has also been suggested as a measure for overcoming the vortex ring state (by the FAA handbook, for example), but it may lead to further loss of height, which has to be taken into account. Another issue is that there are some variations between the different helicopter types- in case of tilt-rotor aircraft like V-22, the only way is to go into forward flight.