If it is first timers, kid are easier but this has work for me with adults as well.

First ask others who do ride if they can tell us how they know how to ride a bicycle

The answer is no, and part of why one cannot forget how to ride a bicycle.  The bicycle knows,

start with ghost riding, run along next to a bike that you are pushing  by the saddle and let go show your pupil that bicycle are self actualizing.  Do this for a while, Make it fun.  Use bikes that wont be harmed with the ensuing crashes.

Next tell your future rider that if they feel they are falling to turn in the direction of the fall and pedal.  

Have them sit on the bike and you hold them from the back of the saddle, don't touch them. 

Now they start to ride the bike and you trot along next to them.  Let go of the saddle as soon as they start and let them ride the bike while tricking them into thinking you are holding them up.  Stay next to them and talk about pedaling while turning into the fall.  When they have ridden a little ways show them your hands and let them know you are not holding them up.  If they fall, point out that you let go all the way back where you  let go.  They road all that way and simply need to try it some more.  

This has worker for me on several occasions.