Segulos – Do they really work? Are they mutar and advisable or ineffective and prohibited?

How do we judge if a segulah is valid or not?
Is the $1 given out by the Lubavitcher Rebbe meant as a segulah?
Are the red bendel (bracelet) and Schlissel Challah actual segulos or prohibited?
Is buying Kol Hanearim, Maftir Yona or Pesicha for Neilah effective?
Are there any valid segulos for parnassah, shidduchim, health, Shalom Bayis and great children?
How do fake segulos catch on?