Is there any document or article regarding the seam best practices.
The most relevant "best practices" are in the seam examples themselves, that are bundled with seam. The wiki is a good example of a well written Seam app. It also has examples of other real-world best practice usage, such as caching, named queries, and even tests.
However, if you are looking for more real documentation, the seam framework website has links to a number of best practices in their KnowledgeBase:
http://sfwk.org/Documentation/KnowledgeBase
I have personally used these docs to do things like slimming down a JBoss server for production and reducing its overall footprint.
Curiously, there isn't much on writing tests (a major "best practice" in my opinion) on this page. :)
If you look for recommendations and tips for a specific topic you find a lot of neat tricks and recommendations in the Seam reference itself. For a lot of topics there are more than one example and they show you how you can do it the most elegant way, e.g. form log messages.
The latest is at http://docs.jboss.org/seam/2.2.0.GA/reference/en-US/html.
I think you you look at the Seam examples you will have a good reference about good practices.