As with every other big event, there were advertisements in a ton of DC titles in the run up to the series.
It’s Coming!
A straightforward logo that appeared on several covers and within issues.
Meanwhile Articles and Checklists
An article in the “Meanwhile” column that Dick Giordano would write. 
An article and checklist of all the issues that crossed over in the series. 
And another checklist that appeared in several issues, helping readers keep track of what was happening.
Weekly Betrayals
An ad pushing the fact that this was a weekly series. 
An ad highlighting the fact that some established character would betray us. 
And another doing the same.
NEW GUARDIANS – New Series
And they’re not going to create those magnificent characters without then doing something with them, are they?
Focus Magazine
A free magazine given away at the time of the series.
Crisis on Earth-Prime 