hmm, i usually open GIMP and fool with the levels, and sometimes channels. i recommend levels if u've got good inked lineart, but if it's a pencil drawing, i usually fiddle around with layer settings and levels. u can also find different tutorials on this on dA (deviantart.com) if that helps.
EDIT: i fooled around a little and this is what i got. it can get a lot better, but u get the point.