Yours truly quoted over in this excellent article by Realm.
The biggest mistake we made with our WatchKit app was including lots of padding around text elements. When designing for desktop and for mobile, we’re used to nice big margins between the edges of our screens and any text written on those screens. However, in WatchKit, if you use a black background, the frame of the watch acts as a natural margin for your content. So butt that text right up against the edge of the screen! It’ll look strange in the simulator, but natural on the device. It has the added benefit of giving you a bit more screen real estate to play with — a very limited resource on the Watch!
The rest of the article is filled with some great tips by some great developers, I’d recommend a read even if you’re not that interested in WatchKit.
Also got me a little mention over at Daring Fireball.