Joshua Blankenship, Design Director at NewSpring Church, has published an article that Authentic Jobs can easily endorse, especially for those seeking positions listed on this site.
In What People Like Me are Looking For in Designer/Developer Portfolios, Joshua doesn’t spare any candor in his remarks and hits all the high points: show lots of work in an easy-to-browse portfolio, include a cover letter, sweat the details, and show passion.
Joshua’s first bit of advice is probably the most notable:
There are 12 year olds with a copy of Photoshop who can set some cool type on an image. Your grandma could learn basic HTML/CSS (hyperbole, maybe—but also probably true). Where’s the proof that you can turn complex thinking into seemingly simple solutions? Proof that you can take a client’s needs and translate them into real work?
If you’re a designer, show a variety of design solutions (include sketches and failed attempts, too—process is important). If you’re a developer, show live code examples (not just “I worked on this website” statements). If you don’t have real clients yet, try your hand at unsolicited redesigns. Just get out there, find problems and solve them.
Having been in hiring positions many times before, I can attest to the need to see one’s complete thinking, not just the end product. Sketches, notes, and failed attempts, as Joshua suggests, move your name to the top of the review list. For more experienced positions, I would even add including any material that demonstrates your involvement in the process of understanding the problem, not just creating the solution — whiteboard photos, an overview of the research you conducted, documents that define the problem, and so forth. (To the extent that you don’t breach confidentiality, of course.)
All of this demonstrates your ability to be a problem definer and not just solution provisioner. You’ll probably have a chance to share this information in the interview, but the supplementary materials you include may be the deciding factor in getting you the interview.
For example, if I were applying for a senior design position and I included screen captures of my work for this very site, I might also include this photo along with a description of it:

This is some of the planning I’ve been doing for upcoming features on Authentic Jobs. Included are comps, data about competitors, goals for the features, and so on. I call this technique the portable war room. It’s just a sheet of butcher paper to which I can tape anything I find or create related to the problem I’m trying to solve. I can pull it off the wall and take it with me to meetings as needed. (For more about this technique, see this article.)
Now, not every hiring manager is the same, and some care initially — and sometimes exclusively, unfortunately — only about the end product. Regardless, it’s probably safe to say that with most potential employers, if your portfolio isn’t stellar, it’s unlikely a hiring manager will take time to review anything else you’ve submitted. Your portfolio and supplementary material get you the interview. Your ability to clearly and confidently define yourself as a problem-solver for the company and its clients gets you the job. So submit your strongest stuff, and then show up ready to sell yourself.
For additional reading, see something I published a few years ago but still remains applicable today: Surviving the all-day tech interview.