When the show is officially announced to TeamAyu members, that'll be kinda your "warning" that the tickets will be onsale to the general public soon. I only get the website, not the release alert e-mails, but a TA member who gets those e-mails will probably post something somewhere about the concert being officially announced.
EDIT: lawl, it looks like the show was officially announced not 5 minutes before i posted this!
Shows how much attention i'm paying.