depends what stains they are really m8
i had some chewin gum on mie, but it was recent, got a tin of that stain remover for car seats. with a built in brush on top.
spray it out and rub like crazy with the brush, worked a treat. then let dry.
if all else fails, try some white spirit, but not too much and dont rub too hard.
Dont take my word for it tho.