If you're interviewing in the traditional way, be sure to prepare thoroughly. Even if the company boasts that they conduct business in a casual environment, dress professionally. Go for a look that couples intelligences with a drive for success. Be careful not to be flashy; you wouldn't want your interviewer to get distracted. Let your resume and portfolio shine, not your tie clip or your earrings.
It's also always important to do sufficient research on the company with which you're interviewing. Potential employers want to know that you're not just out there to get any old job, but that you are pursuing your engineering career with them. Demonstrate that you've seen something special in them and they'll see something special in you.