The Countdown series had its own ads in the form of two cryptic double page spreads that gave a glimpse of what would happen. If I can track down the explanations for them, I’ll put them in but for now, here are the images:
Countdown‘s spinoffs also had more traditional ads running in the comics:
The main series and it’s various one-or-more-shots also had their own ads:
And from memory, this was the first teaser image released, possibly even before the series title was revealed:
Though I seem to remember a version of that with text.