Smartwatch App Design Best Practices

From Five important tips to boost your SmartWatch 2 app development
#1 – Don’t send too much data to the watch When developing for a wearable device, like the SmartWatch 2, it is extra important to consider battery consumption and processing power. For example, sending several full screen images per second will likely create an ever-growing queue of data, and the display on the watch will fall further and further behind. In addition, if the SmartWatch gets too frequent screen updates, it will cause the battery to drain unnecessarily fast.
From The new Android Wear UI- will it pass the social relevancy test?
The context of wearability: constant movement, limited interaction and quick bursts of visual attention.
- Simple: You don't have much canvas to play with. Think useful, not MacGyver or James Bond.
- Glanceable: Users spend 2-4 seconds, so make it visual and based on actions occurring around the user.
- Built around Microinteractions. Microinteractions are those little details that keep a system and a user in "synch"
From Designing for Android Wear
...when designing for these new, tiny screens, one thing we need to keep in mind is to not take the UI paradigms of phones or tablets and expect them to translate the same way on a smartwatch.

"...designing for [smartwatches] should be all about “glanceability.” People should be given a singular and focused interaction when viewing information. ...only the most crucial information is ... shown in a large, easily viewable format."
From In Search of the Next Boom, Developers Cram Their Apps into Smart Watches
“It’s not a replacement for the app on the phone,” says Jacobs of the Runkeeper app for the Galaxy Gear. “It’s a remote control so you can keep your phone in your pocket, can start and stop without pulling your phone out. It’s the same data, the same application, but changes the experience to make it less intrusive.”

"This device is not for long periods in which a person is looking at their watch to consume content" - Pocket’s founder and CEO Nate Weiner

Samsung’s chief product officer, Kevin Packingham said “As we innovate, sometimes we add too much complexity to devices, ... they can interfere with how you live your life on a daily basis. We don’t want that to be the case for our wearables. We want them to be natural, rather than edgy.”

Adam Stroud, Runkeeper’s lead Android developer says "A smart-watch screen is great for display, bad for interaction, There’s not a lot of room for input; the minute you flash up a keyboard on that little screen, it’s over."
From So Far, Smart Watches Are Pretty Dumb
I’ve determined that a good smart watch will have to learn when and how to bother me.
From Encouraging people to wear wearable technology
"...some wearable devices replicate what existing devices can do rather than offering genuinely radical new features."

UX

Android Wear Design Guidelines
A great survey of smartwatch usability UX Review of Galaxy Gear Smartwatch
The key is simplicity (you can do it more easily on a watch) and spontaneity (you can do it right away). Phones already encourage spontaneous use; watches will be for those moments when phones are too big and too slow to access. We’ll need to learn ways to make the apps more direct and distill their essence, so that a quick glimpse on a tiny screen will be enough to get what we need.