When students on the work they've done themselves, instead of on the critiques they get from others, they end up doing a better job improving their abilities later. A student portfolio serves this purpose well because it allows the student to self-identify the work that fits in with whatever characteristics (perseverance, adaptability, cooperation, etc.) they are working on in the classroom. They can spend some time before the conference preparing their portfolio, and writing a reflection on each piece of work they choose to include (I'd figure no more than 2 per subject area per conference). The teacher helps frame the selection process for the student, but allows the student to pick the work they are most proud of.
The portfolio makes for a great focal point in the conference. It's especially helpful for the student who might not know what to say in a conference, since they now have their own self-made presentation material.