The following is a short and almost certainly incomplete list of SOA best practices. Feel free to write your own in the margins.
Do your business architecture work first. Understand the major business processes and how they relate to each other. Map the business processes to the data and systems architectures. Use that knowledge to plan your SOA.
Start small: Build incrementally. Use the first project to build the component infrastructure for your SOA. Add to your SOA on an ad hoc basis, and document and share what you have learned from each SOA project.
When encapsulating existing legacy functionality, use the SOA.
Wire in what you have. Leverage the standards-based interoperability of the SOA to allow you to integrate your application ...