If you have ever built a Windows Store application without browsing to Design Center: http://design.windows.com and Dev Center: http://dev.windows.com then you have missed out on some of the best practices and a wealth of resources you could have leveraged.
These two sites should be the first destination for anyone building Windows Store Apps. These sites include detailed guidance on:
User Experience Design
Design Assets like Adobe Photoshop Files
For developers, the sites can be overwhelming when you first visit them as they contain a lot of information for anyone to absorb on a single visit. I recommend you start at the basic tutorials here:
Or there is a fantastic HOL for building a Contoso Cookbook.
For UX designers, the site provides planning & guidance based on already successful apps.