My tips are just the tip of the iceberg - but surprisingly frequently missed. As with all digital media usability issues, Jakob Nielsen is the definitive source.
Mobile Usability Top Ten Tips:
- Mobile deserves its own design - shrunken web is not acceptable
- Load quickly
- Reduce or avoid images on main pages
- Simplify navigation
- Make buttons BIG
- Condense text - either by writing shorter copy or chunking up into paragraphs
- Make most common info very easy to find (e.g., phone number, directions, event listings)
- Have only horizontal or vertical scrolling, not both
- Make input easy - user input is difficult on mobile devices (other than touching) when possible do things for the user automatically and minimize / avoid the need to type in anything
- KISS - Keep it simple stupid - this is almost always true in all user experiences but in the reduced display of mobile devices and challenging user input, this maxim is even more true
I realize that user testing is rarely down in digital media production, and of course it's always necessary. But considering how absolutely awful and impossible to use some mobile websites and apps are, it often seems like even the developers haven't tried out their designs on a device. And I don't mean test them out on tablets as the user experience on these are much better than on tiny smartphones. Try it out all mobile designs on a mobile device that's a couple years old - as this is what a lot of customers are using.
And if in doubt, consult a professional.