First, keep in mind the constraints of the device. The small screen size on mobile devices means on-screen text or captions will be difficult to read if too small; be sure to make text big. Ditto with in-video links: it's often better to present hyperlinks on the Web page, above or under the video player, rather than in the video itself. A small screen also means wide, far-away shots will be difficult to view: zoom in on the interesting stuff.
Similarly, visitors on mobile devices frequently will hear the audio from your video on small, tinny speakers: be sure to make the audio as clear as possible.
Most importantly, test your video: does it look good on your smartphone? Of course, testing the video for optimal viewing is a best practice regardless of the intended viewing device - but testing on a phone should be a given.