A SharpieŽ works well. I also like the PITT brush pen by Faber-Castell. You just have to make sure the ink dries before you erase pencil lines. I also often use a white PrismacolorŽ pencil or white acrylic paint for highlights.
Two overdue updates here:\r\nThe publicity from this got me a freelance gig designing Netflix\'s holiday mailers for the past three years! \r\nAlso, I just started a kickstarter for my book, \"Pith & Vinegar\". Check it out!