2 mutt questions

First, I have been trying to get Mutt to prepend a character (--) to
each line of a message I reply to. I thought it was the "set indent_string"
but that does not appear to work as expected.

Secondly, how can I get Mutt to order the messages by thread each time
I open Mutt. Every time I open Mutt, I have to reset the order of the messages.
More an annoyance than anything.

v/r
-Mike