Usually it's based off a specific idea or goal (though Subdivisions was me just winging it) that I have for the map. Typically I plan it out in my head, and then use my photographic memory to transfer it to what I want to it be in-game. Although with that said, I do change ideas based on items available, map size, budget etc.
Yes i do plan, some tips would to make it new, clean, FUN, and try to mix the objects you use to not make something so constent, for ex: making the whole roof, floor, and walls all dubble walls. Also try to think of the gametype you want to use the map for. Think about the gametype settings before you start forging.