Err, I don't necessarily plan everything out beforehand, because you can't know exactly when and where you'll run into animals/chasms/veins/other features that can be major factors in the fortress proper- even if you use regional prospector. (which I do)
I do like to start by designation, though. First I choose a large overland area, and designate it for clearcutting and plant-gathering. Then I build into the soil three small rooms, a bedroom/diningroom, farm plot and storage chamber. Once that's done I start planning out the fortress proper.
Usually I have a wide open area in the center of my fortress for production with massive stockpiles in the center and workshops in chambers all around. This means all production is generally in the center of the fortress and as close to everything else as it can be. Kobolds/Goblins also have to travel through highly populated areas to get near my children/valuables. To this day I haven't lost a single item/child to theft (using this strategy). There's a small room dividing that area and the apartment complex (fractal bedchambers, always cool once fully smoothed/engraved) that I use for stone dumping- since right now dumps can contain massive amounts of stone in single spaces, I can undesignate the stone I want to use and suddenly it becomes a compact stockpile.
After that I'm usually nearing 80 dwarves, so I build some fortifications, survive the onslaught and grow from there.
Also, it's incredibly beneficial to build a mining shaft somewhere near the middle of your fortress. You get a good idea of the layers of rock beneath you and if you don't happen to have a chasm, that's useful. Any time I hit Gabbro/Bauxite or other useful layers I go into super mining mode.
Also, your first six dwarves, in my experience, should be Miners/Warriors, with the last an Appraiser/Organizer/Social Skills Guy. This allows you to spend the first year burrowing out your fortress proper, and once help arrives you have a six dwarf guard with obscene stats.
I have no idea how much of this helps you, but I felt like writing it all out, heh.
[ June 14, 2008: Message edited by: DarkMagnus ]