I am using http://blogs.microsoft.co.il/blogs/justguy/archive/2009/01/19/wpf-combobox-with-checkboxes-as-items-it-will-even-update-on-the-fly.aspx?CommentPosted=true#comment
By using a DynamicResourceinstead of StaticResource, it will find the resource at runtime, so you don't have to worry about the order of the declarations in the XAML.
There may be downsides to this (?), but in particularly sticky cases, this has been the answer to my prayers.