I designate everything before digging anything so there are no mistakes or wasted space.
I'm really into making stair step like entrance halls down ward then making a huge dining/meeting hall. This becomes the heart of my fortress. I try to make it 3 z levels. I make a kitchen with food stockpile then a still with drink stockpile, both near the center. I then design hallways branching out from the center great hall 2 wide. Sometimes a 4 wide or 3 wide main hall. I try to make up down stairwells where they intersect and then areas for bottle necking enemies into traps and ballistas.
Living space is planned next. Anticipating traffic, I will make multiple connecting hallways at right angles. I give each dwarf a 2x3 room so they feel like a king. Living space for important people is reserved deeper into the wall opposite the main stair/entry halls.
2 z levels under this becomes the great public tomb. Every dwarf gets a cubby hole and a rock coffin. A large section is reserved for important peoples personal tomes. Pet cemeteries are just extended down the hallways with a similar basic set up.
I realize I have a doctor dwarf by now and he needs a hospital, so above the living space I make hospitals and stockpile areas for medical furniture and supplies. The dwarfs get 4x4 rooms for this. they can fit a bed, table and traction bench with one square left over.
I begin planing my workshop area now which is just below my great hall, and then put the jail and animal areas below the tombs. Most crafting rooms are just 2 12x15 areas with a stockpile area betwixt them with long halls separating the stocks and the shops.
I then find magma for furnaces and make it under all that, sometimes just past the first cavern layer.
I then quit after 3 hours of meticulous obsession, because something happens right after, !!FUN!! or otherwise (computer crashes, dwarf fortress freezes solid, battery of laptop dies because I didn't notice it was unplugged) and I lose everything...