What have you been using to clean them with so far, and what exactly are we talking about here? I have a black dash myself and some Armor All never hurt.
Yer, Amour All is good for getting the marks off and then use an interior cleaner/polish to give the plastic some moisture. It's what detailers use on dashes etc.
It's like white marks on high use areas such as gear stick, steering wheel and inner door handle. I was told it was build up from make-up, dirt etc from my hands. I think I'll start with Amour All and see how I go.