Hubee:
The 5th traditionally had a 25 or under rush, but I haven't seen it advertised recently.
Your best bet is to become a "Friend" of the 5th Ave through their FaceBook page, and they'll sometimes post up specials that they have on there, but it's usually "the day of" specials that don't have all the time, and usually for low traffic days (Tuesday and Wednesday nights)
There also used to be a "day of show" tickets window in the Pacific Place in downtown Seattle, but they closed this last month, and I don't know if there is anywhere else in Seattle where they do that.
Xanadu! Can't cry on cue!