Personally I use following repository structure:
/project
/trunk
/tags
/builds
/PA
/A
/B
/releases
/AR
/BR
/RC
/ST
/branches
/experimental
/maintenance
/versions
/platforms
/releases
There is also a diagram illustrating how those directories are used. Also there is specific version numbering approach I use. It plays significant role in repository structuring. Recently I have developed training dedicated to Software Configuration Management where I describe version numbering approach and why exactly this repository structure is the best. Here are presentation slides.
There is also my answer on the question about 'Multiple SVN Repositories vs single company repository'. It might be helpful as long as you address this aspect of repository structuring in your question.
