what I do. very basic for a long pose 3hr - 5hr
start with the basic layin of the figure, spending an hour, or two sittings? on this. (this includes getting proportions correct, no shading, no values, just lightly drawn lines (so you can erase alot), i draw using mostly straight lines at this stage, (which will eventually turn into curves the more straights you put in) drawing with s/c curves is to difficult for me to messure correctly.
Just remember, everything you'll be basing off of is in this stage, so its the MOST important stage of all. if you fly through this part too quickly, you'll be fighting your drawing the whole way through. (mistakes become VERY apparent when you start to block in your shadows)
step2, im still doing more layin getting my forms to read correctly
step3, I'll block in shadow shapes, keeping it at 2 values (the white of the paper(brown incase of newsprint), and a middle dark value for shadows (so i can edit where need be BEFORE i throw in the darkest darks (or again i'll be fighting the paper)
step4 introduce my darkest dark, and mid tone. so thats only 4 values .
after that, its up to you how you want to finish i guess, Im still struggling with this each drawing, and alot of the times get to complicated. (especially these days, under a natural light)
anyhow, thats just the way i've been handling long poses. and let me know if that even makes any sense, i've never tried to put it in words before, just trying to remember what was taught to me. and i might have missed some steps in there...
OH and make sure you step back alot while doing a long pose. it's like a whole new drawing after sitting so close to it for so long, (and it'll prolly look pretty out of wack and ugly)