Not knowing about the company's design style, values, and previous work may make you appear to be unprepared. Research the company in advance.
Including too many projects overwhelms the interviewer. Choose a couple of strong, varied pieces that demonstrate your skills and style.
During design challenges, failing to approach the brief well can mean missed opportunities. Ensure you get across the problem and solution needs prior to starting your design.
When showing your portfolio, don't get too defensive when asked or when you receive criticism. Learn to be critical and defend your thought process appropriately.
While it is desirable to be technically skilled on design tools, don't forget to highlight your creativity, problem-solving, and design thinking process.
Good communication is paramount. Failing to clearly communicate your design rationale or failing to listen to interview questions can dash your hopes.
Failing to practice responses for frequent questions or lacking a design problem in hand can make you seem unprepared. Rehearse your responses and be prepared for impromptu tasks.