A very good guide that explains exactly what to do, and how to get inspiration to do it right:

  1. Format:
  • Device specific – one for the iPhone version and one for the iPad version
  • Different resolutions for each device: 640×1136 (or 1136×640 for landscape mode); 900×1200 for the iPad (or 1200×900 for landscape mode) 1080×1920 (or 1920×1080 for landscape mode) acceptable.
  • Maximum length - 30 seconds.
  • Device-captured footage – no live video, no fingers, no devices shown.
  • One language localization – try to keep it in English.
  • Disclose in-app purchases – if applicible.
  • Gives a true experience of your app.
  • Adding text and voice over is OK – but not too sales-y.
  • Background music – do it right.
  1. Guidlines: Respect them. Play it safe.

  2. Get inspired - learn from others, get ideas. a few examples:
    http://vimeo.com/102510948
    http://vimeo.com/99357281
    http://vimeo.com/100998905

  3. Plan:

  • Know what is your audience and goal – focus only on them.
  • Write a script for the video. Don't improvize.
  • Show the right data – make sure you show the best gameplay or data that a frequent user would have.
  1. Produce.
  2. Optimize - check for conversions, looks at app reviews and ratings, always look for places ou can improve the video to match the taste of your users.

Awesome tips. Worked out pretty well for me.

Full article:
https://www.apptweak.com/learn/app-previews-6-advices-to-produce-videos-for-the-ios-8-app-store/