Simple: blog on everything you do in preparation, including multiple initial ideas. If you've considered an idea and rejected it, briefly post on this and explain why you didn't pursue the idea (including any points I've raised with you).
Your pitch will be filmed, including Q+A, so you can upload this and reflect on the pitch afterwards.
PREPARATION: RESEARCH + GATHERING VISUAL MATERIAL
If your pitch fails to evidence sufficient preparation you will be asked to do it again until it is clear that you have undertaken considerable work on this. It is not acceptable to expect to simply ride on the coat-tails of someone else's diligence and effort.The simplest way to approach this is to consider what sort of questions I might ask, and come prepared to answer them - with visual material which illustrates your idea.
I will obviously need to know you've undertaken initial research into the genre, act and track: viewed examples of genre vids and act vids, plus made sure you're clear on how your idea fundamentally differs from any existing vid. You shouldn't be pitching for a vid for a track which has an existing vid from the past 10 years.
You will have researched the lyrics and anything thats been written about them; it doesn't matter if your idea is for a concept video, you still need to understand the nature of the track you propose to work on.
You will be aware of any directors this act have frequently worked with, and some features that commonly occur in their videos.
You will provide visual material rather than rely wholly on words. This means images or basic footage of possible locations, costume, characters/cast, props etc.
You will show awareness of what you need for this idea to be achieved.
You will show that you have thought through the idea, and are clear on what core/primary and secondary audiences it will be aimed at, explaining how your idea fits for these. Noting intertextuality can be useful.
You will reference existing videos to illustrate and justify your ideas. If you are seeking to challenge conventions reference conventional vids.
You will have looked over the assessment criteria so you're sure your idea will lead to a very high mark
You will explain why your idea will lead to a video that target audiences will happily watch repeatedly (music vids are designed for multiple viewing unlike films), and be clear on the variety of locations and events plus any mix of performance and narrative/concept which will aid this.
DELIVERING THE PITCH
You've a lot of info to convey so should be taking around 2mins for this. That includes time to play part of the track so we're clear on what you're pitching.
You will be stood by the projected image so whatever you create should not require clicking which will distract you or the audience (including me) from the pitch.
That rather implies a film file or a PowerPoint with timings built-in, though PowerPoints can be tricky with audio/video.
I've written much more on this in the past, but the above hopefully covers the key points.
See also http://musividz.blogspot.co.uk/2012/05/hwk-pitch-for-class-mvid.html; http://musividz.blogspot.co.uk/2011/09/pitching-your-proposed-coursework-track.html; http://musividz.blogspot.co.uk/2010/11/coursework-pitches-draft.html.