Here's an interesting tip. Or is it complete misinformation? It *seems* to work this way, but I could be all wrong...

Let's say you've drawn a nice little door icon, with animations for when it opens and closes. When it's open, you want the players to be able to see the floor beyond. However, there's a catch: the map editor is smart, and if you lay down a turf without any transparency, it will remove all the underlays (and rightly so). 99% of the time it's not a big deal.

For the other 1% of the time, you can use The Magic Dot. Just go to the icon state that's used when you're actually placing the turf on the map, and put a little transparent pixel somewhere on it. Ta-da! The underlays remain intact.