This is one of the posts I only write for myself (to not forget were I found the information). I this case I just found a very just but interesting list of best practices for XAML code from Kunal Chowdhury. He is an MVP in Silverlight. But before I write to much. Just follow this link to read it.
http://www.kunal-chowdhury.com/2010/08/some-best-practices-for-silverlight.html