[Avodah] "Ancient" Minhagim

[Harvey Benton]

R. Kenneth Miller wrote:

R"n Toby Katz wrote:
> Well as you know, I do not look kindly on ancient minhagim
> that are less than thirty years old. Get back to me when
> women have been reading ketubot at weddings for a hundred
> years or me, and I'll rethink it.  Right  now, I totally do
> not believe that this is an "eilu ve'eilu" issue or a
> matter of different minhagim in different communities,

R. Micha Berger wrote:
How big of a breach
can mimetic halakhah absorb and still survive to guide us.

HB: 
In general, throughout history, what has guided the Jewish Nation in it's adoption and eventual acceptance of new Minhagim??  The Arizal had many innovations, as did Chasidus.  Speaking Yiddish (though probably not a Minhag) and considering it a Heilige Language does not go back to Har Sinai.  Kenneth Miller mentioned silver Ataras on Taleisim, etc.

How are we supposed to know what is a valid practice, and what is a Minhag Shtus?  Do we only find out about a Minhag's validity after a 100 years of its use??

Good Shabbas and Go Israel!!