Ideally, the video clips in a portfolio come from the filmmaker's past works. It's a matter of choosing bits and pieces of impressive shots that could make the viewer interested in his artistic taste and filmmaking skills.
Like a resume, a film portfolio should fit the kind of work the filmmaker is aiming for. If it's for a horror film, it's best to showcase works with good horror elements. If it's for a TV commercial, he can't just choose dark and rough independent film-style scenes no matter how technically good they are. Commercials sell products, so advertising people want to see glossy video and product shots in a portfolio.
As an artist, a filmmaker is best remembered through his most striking work. He is also better remembered when associated with a particular style or genre. In such a competitive industry, having a niche and good packaging go side by side with an accomplished film portfolio. These are essential in acquiring that needed "recall" from producers, clients and production outfits.
A film portfolio should be short but striking. Ideally, it should not be longer than three minutes. The people who will view it are usually busy executives who need to view dozens of portfolios from different directors. For such a creative job, the audiovisual skills in telling a short but superb story are always crucial.
In editing the portfolio, only the best of the best shots should be included. There shouldn't be much edits other than the cut-to-cut of one footage to the next, or any applicable transition shots and minor effects to enhance the reel. Each chosen shot should still match how the original footage looks. The client or producer could possibly ask for a full copy of the director's sample work as seen in the portfolio. It is also important to put suitable, upbeat music to it. Editing this kind of video is similar to the editing of a music video.
For a reel sent in DVD format, it's best to come up with a professional-looking sticker cover and DVD casing complete with art-directed collaterals. This is very helpful when there are countless DVDs sent to clients and producers and there are chances that they skip watching some of them. Most likely, good packaging can help a portfolio make its way to a viewing.
It is important that the director be prepared to answer all possible questions about the work in her portfolio. When an impressed client or producer asks any technical, thematic, creative or other production-related questions, a bad answer might just lead the supposed positive impression into complete disappointment.
It is also ideal to post a copy of the portfolio online. And it is best to supply enough information about the reel online to make a more lasting impression about the filmmaker's accomplishments.