Your portfolio is your most powerful business development tool, and most designers underinvest in it. Not in terms of the work it contains — most designers care deeply about that — but in terms of how the work is presented and what story it tells. A portfolio full of strong work shown badly is still a weak portfolio. Context, curation and craft all matter.
The most common mistake is including too much. Clients and creative directors are busy; they will make a judgement about your abilities based on the first three to five pieces they see. If those pieces are your best work, you are in good shape. If they are middling projects you included because you wanted to show range, you have already lost the room. Edit ruthlessly and let your strongest work lead.
Case studies beat image galleries. Rather than showing a finished design in isolation, walk the viewer through the problem, your thinking, and the outcome. This demonstrates not just what you can produce but how you think — and how you think is ultimately what a client is hiring. Process matters. Show it.