Last night, Apple published a webpage detailing the 'Made for Apple Watch' program, which will allow third-party companies to make custom bands for the Watch.
Apple Watch bands are easily changed with simple release buttons and lugs that secure the band to the Apple Watch case. When creating your custom bands, refer to the band design guidelines and lug profile. Apple Watch lugs will be available soon through the Made for Apple Watch program.
In two documents available on the page, Apple has detailed band design guidelines and the Apple Watch lug profile. If you're curious to know more, the band design guidelines include some interesting tidbits on the materials, measures, and design Apple recommends for third-party bands.
Also, Apple posted an update to the Apple Watch Human Interface Guidelines first released in November 2014. If you're a designer or developer making apps for the Apple Watch, make sure to check out the updated document (and downloadable PSD/Sketch files) here.