this is the filter I use for descreening http://www.descreen.net/eng/soft/descreen/descreen.htm
I always scan at 600ppi or higher, and then resize down to 300ppi
600ppi, descreen, resize
when I scan I turn ALL scanner software shit off. no automatic descreening, color correction, etc.

and then the only thing I do besides descreen is set the black and white levels
so the best method for this:
make a copy of the image
find an area of the copied image that you believe the printer printed as "pure black"
magic wand that area to select it, filter > blur > average
now that area is all the same color. repeat this for a pure white section
layer > new adjustment layer > levels
select the black eyedropper on adjustment layer, click it in the averaged black area
repeat for white
duplicate that levels layer to the original image (the one you didn't do any Averaging on)
if you average out the color of something that is black, that average color is, in theory, black

if you're monitor's any good you can see some good colors from this process here http://gamepreservation.tumblr.com/post/108855176394/gamefan-april-1999-volume-7-issue-4 and here http://gamepreservation.tumblr.com/post/106965022454/heres-a-rare-promotional-comic-book-for-hudsons

if you credit me though maybe you should say it's adapted from this guy's methods https://twitter.com/AndrewPerti
he goes a little crazier than I do
maybe I should just write this up somewhere

it's a compromise between automated levels and manual methods that
take an unreasonable amount of time to do right
it's good enough for me. also it's its own layer on the original image, so you can always go back and tweak it
I think adjusting hue and saturation is adding information to the scan that isn't on the original image