Guarded by triggers are great! Correctly using them makes the AI much more effective, and they are super-easy to boot.
You place them, then put units on 'guard' waypoints. Once fighting breaks out or enemies are spotted near the triggers (their size doesn't matter I've heard), then units on 'guard' waypoints will go to the area to help out. Units on 'sentry' waypoints also use the triggers, although i'm not sure how it is different than 'guard' wps (maybe they return to their original WP after checking out the trigger).
I don't really know how it works any more specifically than that, but I do remember seeing a post here somewhere a while back that described them in great detail. I'd suggest a search from the beginnings of these forums.