Groups are an AWESOME compartmentalization tool. Another one I'm fond of is using just an absurd amount of event sheets for further de-tangling. All my UI stuff is in my UI evSheet. The one downside is I keep forgetting to include the new event sheets in to the gameloop's main event sheet, so I'll spend twenty minutes coding something, try it out, and have it not work, then spend twenty minutes going through my own code trying to figure out what I messed up, until someone watching my stream goes "Hey Brendan, remember when you told us to remind you to check if you included the event sheet? Well, did you remember to include the event sheet?"
I then bang my head on my desk, fix the problem, and get back to coding. xD