Don't wear dress shoes with jeans. Skirts high above the knee send an unintended and unprofessional message. Keep your hair in a current complimentary style, any length, but not dangling over your face. Men, please don't comb your hair over the thin spot on top. Thin is OK, and shaved heads look terrific.
Bad teeth are a huge turnoff.
People look at your mouth when you talk. Take care of your teeth. Rumpled, worn, faded, or stained clothes suggest you're not in control. Buy the best fabrics you can afford. Give them the surreptitious twist test when the salesperson isn't looking.