Every map i've built, I have had at least a simple sketch before I go into forge. Typically I draw out my major paths and lines of sight, and maybe a few notes in with it about a tac jump, or some other feature, but often those are added later. Then I go into forge and set out a basic layout so that I can look it over, and get some size dimensions and make sure what I was planning will work, or find that the size needs changed. I think if your not planning out your map, then your un-likely to come out with a good design. A bit of prior thinking will often reveal that something won't work, or will show flaws that can be fixed up, and thus save time in the building stage. And no, the excuse "but i'm not good at drawing" is not a good reason, because i completely suck at drawing and can still manage to get a decent sketch of what I want to do with the map. My closing thought would be, Would you go to build a house without first knowing where you want the different rooms to be?
Typically I only Forge when I get an idea. From that idea I start planning things out in my head about how I want the basic design/layout to look. Eventually Ill end up drawing up a basic sketch to get the shape down, maybe add in a few details, then start Forging it. After I get the basic layout down, Ill go around and look for places when I can place aesthetic pieces of LoS breakers if theres a particularly long and irritating LoS, and anything else I think it needs. Alternatively, Ill get an idea, go online to get inspiration from screenshots of various things (unusual/interesting architectural designs help a lot), then use certain aspects of any images I find in certain areas of a map. I've done this for a map that I was working on a month or two ago, that I probably should get back to. :\
I was about to say that! The thing is, forging without a plan is not really comparable to building in reality, because it's easy and costs nothing to make radical changes, demolish and replace whole sections, etc. The reason I frequently forge with little or no plan is that it's as easy for me to come up with layout ideas in forge as it is by sketching ideas, using a program like Sketchup, etc. Forging may as well be the same as drawing to me, except that I can much more easily work in three dimensions, with the object limitations and budget built right in so even as I'm moving things around and letting my plans evolve, I know what I can and can't pull off. Something like Sketchup seems basically redundant to me - I'd only bother with it for the benefit of planning on my computer during times that I literally can't fire up my XBox. Otherwise, why not just do it in forge itself? I think where people get into trouble with forging without a plan is when they get married to whatever they've already plopped down. To use forge as both a planning and building tool, you really have to be willing to mess some stuff up - to rebuild an entire map if it's in the wrong place; do rough drafts that may bear little resemblance to the final version; destroy whole sections and build them again in an entirely different way; etc. I also do a lot of proof-of-concept builds away from my map (e.g. "what would this kind of hallway look like with a turn, a 25 degree incline, and a drop hole in the middle of the floor?") and then transplant them if they work out like I had hoped. And finally, you have to have good instincts for overall map layout, or at least be willing to sometimes fly back to get a bird's eye view and see how things are all fitting together. What you DON'T want to do is make a big box (or just start in the coliseum or quarry) and put some stuff down until the space is filled up. That's no way to make a coherent map.
I just choose a piece that I want to design my map around and go from there. Sometimes I'll doodle a quick idea or two just to get get some ideas flowing, but once I'm in forge I build on the fly and maybe one idea from my drawing sticks. I feel more creative when there is minimal planning.
Personally, what I do is draw a basic layout of the map on a sheet of graph paper. then take sketching paper and make little adjustments, then pile the papers on top of each other and I see my map as a whole.
Never have, probably never will. I always get this S00per awesome idea in my head that drives me to boot up the 'box and Forge, then I just build until I like how the whole thing is.