Source: How to define default values for pages and blocks in EPiServer CMS 7
Also see this: using [DefaultValue] attribute:
Default property values on content in EPiServer are set by overriding SetDefaultValues. In an attempt to make my model classes a bit tidier, by not having to do the override in lots of places, I wrote a little piece of code which enables the use of the DefaultValue attribute (found in System.Component)
from: http://world.episerver.com/blogs/Per-Magne-Skuseth/Dates/2014/3/Attribute-based-default-values/