How to Make a Good Expansion–tips from a designer of Alcatraz:

http://boardgamegeek.com/blogpost/12605/designer-diary-alcatraz-maximum-security-or-how-to

SEE ALSO:

http://cardboardedison.tumblr.com/post/30466346166/some-principles-for-creating-an-expansion-from

http://cardboardedison.tumblr.com/post/19177839103/lessons-learned-designing-an-expansion-by