HOW TO: Use Medium Trust in ASP.NET
Solution
How To: Use Medium Trust in ASP.NET 2.0
by MSDN - configuring medium trust, locking trust level, and optionally creating custom policy based on medium trust.
Hosting ASP.NET Applications in Medium Trust
by Scott Mitchell (4GuysFromRolla.com) - also includes working around medium trust limitations.
Related Resources
MSDN: Security Guidelines: ASP.NET 2.0
Medium Trust Issue in ASP.NET 2.0
by Keyvan Nayyeri