I keep 'head' workers for each work function. (Head cook, head Armorsmith, head siege engineer etc.) and allow each person to 'take on' two lackies, depending on who comes in migrations (exception; farmers, cooks, and brewers). The head 'whatever' has no hauling jobs, and the two lackies have hauling jobs related to their field. Anyone of a chosen field (except peasents) that exceeds that is put into my 'system acceleration/response team', is given masonary, architecture, carpentry, mining(when needed) and is drafted into the military. They train until I need some mass construction/excavation. This means if I, say, wanted to build a structure, my regular masons would mass spam blocks while my mass construction team took care of moving all building materials to the sight, then subsequently making the structure.
The peasents, however, are a different matter. I keep my peasents for all my major hauling jobs, which, in any large fort, feels like 90% of the work...
Peasents are usually just general haulers, but I always keep some food haulers, spanning out about 3 per 50 dwarves. You can also keep lots of food haulers and make your food industry lackies as dedicated as the mains.
This system seems to work fine for up to 300 dwarves, though at that point, you usually take on two mains per industry with four lackies. I haven't tested anything over that, though.
Have fun!