Did you know that there are design templates that can be used to create mockups of Windows Universal Apps?

https://dev.windows.com/en-us/design/assets

In addition to the templates for PowerPoint and Adobe Illustrator the now provide Adobe Photoshop Templates as well.

Check out this link for details on the new Photoshop templates:

http://blogs.windows.com/buildingapps/2015/10/23/windows-10-uwp-app-templates-for-adobe-photoshop/

On a side note, check out this great collection of 72 design ideas for Windows Store Apps (Windows 8.1 for now). They include full source code and assets that can be downloaded from Codeplex:

http://www.timmykokke.com/2013/02/50-design-templates-for-windows-store-apps/

And here is the full list on Codeplex: http://www.codeplex.com/site/search?query=win8template