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:
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. 🙂