Trunk / branch best practices

Hi

I've been searching for some answers regarding my scenario.

We use subversion and following structure

/svn/trunk/sln1
/svn/trunk/sln2
/svn/trunk/sln3
/svn/trunk/sln4
/svn/trunk/externalDlls

where sln3 and sln4 depends on dll's from sln 1 and 2, so when we checkout trunk the external folder is in the trunk root.

When we branch sln3 ( or 4 ), we also have to branch sln1 and sln2 so the directory struncture is similar to this

/svn/branches/sln3/sln1
/svn/branches/sln3/sln2
/svn/branches/sln3/sln3
/svn/branches/sln3/ExternalDlls

What is the best parctices implementing this structure in teamcity?  or Should we use some other structure?