I tend to dig a large courtyard/front area along with an initial set of 'starter' rooms to get the fort off the ground, then dig further back in order to fan out a bit more and build those nice big feasting halls I love so much.
the courtyard basically serves as a dump and a trade depot area (they're the same thing, right?) Since I might need the bones from the refuse dump in the event of a siege to make bolts. Usually I have a secondary barrier behind the courtyard as well as a mechanism for inflicting some sort of unpleasantness on the general area of the courtyard in case it's overrun. Later I like to throw up an outdoor barracks for a few guards, so I have a few outside all the time.
On the outward side of the courtyard I dig a moat, usually three tiles wide and maybe 2 or 3 z-levels deep, just for the feel of it, with a drawbridge across it and a fortified wall next to the moat. This gives me a nice initia defensive position and lets me route any incoming hordes into my field of doom and cages and also possibly lava.
For that added touch of unpleasantness, I also like to make the bottom of the moat a fat depot. Quite unpleasant to fall into and nearly impossible to climb out of
Of course, this does tend to vary based on terrain, how I feel at the time, and all that, but the initial drawbridge/moat is good for stopping most attacks, allowing my crossbowdorfs to pick off the attackers while they mill around in front of the bridge looking confused and shouting about crazy Dwarf bridge summoners.
This design also has the advantage of keeping Elves out of your fort's interior, as it's a well known fact that it's nearly impossible to get the smell of Elves out of anything.
Sometimes I develop the exterior bit into a castle-like structure, while other times I let it remain as just a stockade across the back of the moat. It's all a matter of choice