I usually separate my fort by levels, around a central stairway. I don't think I've ever lost more than a half a dozen dwarves due to falling down stairways in the 5+ years I've been playing DF. I find spiral ramps to be a pain in the backside to lay out. I guess need to use macros more.
- Depot is usually just below ground, with nearby stockpiles for stuff to be traded. Finished goods, low quality mechanisms, statues and furniture.
- In the lowest dirt level are my farms/pastures, plus farmers' workshops.
- Immediately below the farms is the kitchen/still with food stockpiles. Butcher, tanner and fishery are also here.
- Below the kitchens is the main dining hall with its food and booze stockpiles that take from the ones buy the kitchens/stills.. Hospital, library, and temples typically go here as well. Tavern may be here or may be up near the depot.
- The next two levels are for bedrooms. Trying to put them all on one level makes for some long walks to get to the central stairs.
- Next is a level for stockpiles of stuff that will be used inside the fort. Good quality Furniture, mechanisms and statues, plus a big wood stockpile to provide more than enough to get me through a siege.
- Then is my general non-forge workshop level. Each shop is surrounded by its input stockpile.
- Next is my forges. Here are wood burners, smelters, forges, glass furnaces and kilns, along with the appropriate stockpiles to keep them fed. The wood furnaces have a medium stockpile that takes from the one above the workshops.
- Right below the forges is reserved for magma plumbing.
- Somewhere below the forges is where I put my tombs,usually two levels below, but sometimes deeper if there is a marble layer close.
I'm not claiming this is optimal, but it works out pretty well. I assume that hauling labor is cheap after the fort reaches 30 or so dwarves, so I don't mind having shop output have to get hauled a ways, but I always want the inputs to be very close to the shop so the skilled labor doesn't waste time carrying stuff very far.
I use hotkeys to move around:
F1 - Surface
F2 - Depot
F3 - Kitchens
F4 - Dining
F5 - Bedrooms (top floor)
F6 - Workshops
F7 - Forges
Shift-F1 - Marble Quarry
Shift-FN - Other points, such as particular ore mining areas, or the level where I'm tapping into the magma sea.
Even if I don't use the same layout as described above, I always use the same hotkeys for F1-F7, since I've got it in muscle memory to use those keys to get to those areas. One fort I'm playing right now has the kitchens and dining room on the same floor, but I still have F3 and F4 both set to go to the appropriate places on that floor.
One problem I find is that this layout works so well for me that I have a hard time getting myself to do something else, which tends to make one fort much like another and leads to boredom. So I'm working on trying different plans to keep things interesting.