What are some tips for impressing employers in co-op and job interviews?

Some of the best advice I ever received was to always smile and don't forget to be yourself. Outside of this, it is always a good idea to research the company/job that you will be interviewing for. Know the job duties and the skill set necessary to excel in the role. Be prepared to explain why you are the best candidate for the role and what makes you stand out from other interested parties that are interviewing for the same position. There are a ton of wonderful resources readily available on the internet. Search things such as: "Top interview tips", "interview skills" or "how to ace an interview". One great resource is the Forbes website. Best of luck on your interviews!

http://www.forbes.com/forbes/welcome/?toURL=http://www.forbes.com/sites/jonyoushaei/2014/10/20/12-surprising-job-interview-tips/&refURL=https://www.google.com/&referrer=https://www.google.com/

In addition to what has been said above, I would say be visibly excited about the opportunity and eager to lean as much as possible. And do not worry so much if you are good enough for the internship or not, let the interviewee decide. Prepare for the interview, know your resume and be ready to talk about the things you have done in the past, definitely research the company, and try to have fun with it. In many cases, it takes number of interviews to get it right, or even get the internship or a co-op job. Do the best you can, and no matter what, count it as a learning experience that will help you in the future. Good luck!

Apart from your knowledge/expertise in the field , softs skill are important. Communication, Confidence, Body Language, Maintaining Positive Attitude matters.
There are many do's and don'ts available on internet. You can go through and start working on it.

Shaye:

I have conducted many interviews during my Engineering career. There are several things that have impressed me about a prospective candidate that I can share with you:

The candidate has actively served in a campus or volunteer organization that has performed major volunteer work, particularly if this work involved engineering. For instance, Engineers without Borders has chapters at several hundred colleges. Its members design and carry out infrastructure projects in the U.S. and in dozens of foreign countries. Active involvement in organizations such as this make a candidate really stand out.

A senior project (or a thesis research project for an advanced degree) make a candidate stand out. This experience demonstrates that the candidate is capable of designing and carrying out sophisticated and self-motivated work.

In both cases these types of experience make for great talking point during the interview and offer a chance for you to really shine!

Good luck, Pete Sturtevant, PE