The best thing to do is to really get to know your characters. That way, you'll be able to sufficiently get into their heads to make their voices sound different. I'll second limyaael's suggestion of working through questions about their backgrounds and personality.
A recommendation - Sarah Monette does separate POVs fantastically well in Melusine and The Virtu (though she only has two different ones). Not only are they extremely distinct, you can also learn a lot about the "speakers" by the way Monette has them "talk".