planning is for pussies! just kidding =) I usually hop into forge mode with the intentions of wanting to forge something. As ideas continuously pops in my head, the map begans to unfold into something that i'll most likely spend hours perfecting.
I go into forge with a specific idea or plan in mind, then just let the style of what Im building flow on its own. My maps that I actually fully plan out outside of forge usually end up not finished, or just not being appealing to the eye.
I find planning out maps to be harder, but more successful than going in blindly. Going into forge blind usually never works for me.