Establish data archival and retention requirements so that they can be used in understanding data storage needs.
If possible, load a test database with typical project sizes and complexities to better understand how your database will grow as projects are added to it. Don't forget to publish the projects, set the availability and OLAP calculation window time frames, and build the cube.
Expect system performance to change over time; enact regular system monitoring and logging to address performance needs proactively.
Monitor WSS database growth carefully because user acceptance of WSS can sometimes vastly surpass any expectations.