Honda Civic sometimes starts and sometimes doesnt

I have started my car in mornings and went shopping. After coming back to
car, it wouldn't start,  This has happened several times wherever I go and
come back to car, it won't start.  I took it to my mechanic and he said for him
to check ignition, my car would have to NOT Start, so I left it with him, and he
said everytime he went out, the car started, so he couldn't help me.  But I
can't just go out anywhere, then when I get back into my car, it won't start.  
That is frustrating.  There must be something someone can do.