I use a dual-trap setup and I make all my paths twist back and forth to make them longer in a small amount of space. I also put walls in all the corners so that the path is always exactly one tile wide and they have nowhere to dodge except the pits.
The first trap only has a 1z drop. It's deliberately non-lethal, so that I can funnel the enemies down to the lower layers and keep the upper parts clean. Once you fall down it, your only way out is to go over a second dodge trap, which leads up the stairs (that gets you to the middle of the upper level of dodge traps. This secondary trap is very deep and generally lethal.
I also put a few cage traps at the end where the trap actually leads into my fortress, not that anything gets that far. And a few lethal traps just before that. The far end has war dogs, who munch on the kobolds and any other trapavoid nasties that try to get through. I suppose that if I were designing it on a mature fort, I would also include some lever (or repeater)-controlled spike traps at key points. Just because there's no kill like overkill.
I should also mention that, near the bottom of the pits, I have fortifications one z-level above the bottom, where I can station marksdwarves to finish off any goblins who are not quite dead, but also not quite alive enough to make another run through the gauntlet. And there are drawbridges that allow access so that I can claim all the goblinite. Not to mention Dwarven Bathtubs to clean off any grime people would otherwise get all over my nice clean fortress.
Yeah.... I use a fairly involved setup.