If you follow my recommendations below (I have for years), you will be able to:
-- put each project anywhere in source control, as long as you preserve the structure from the project root directory on down
-- build each project anywhere on any machine, with minimum risk and minimum preparation
-- build each project completely stand-alone, as long as you have access to its binary dependencies (local "library" and "output" directories)
-- build and work with any combination of projects, since they are independent
-- build and work with multiple copies/versions of a single project, since they are independent
-- avoid cluttering your source control repository with generated files or libraries
I recommend (here's the beef):
Define each project to produce a single primary deliverable, such as an .DLL, .EXE, or .JAR (default with Visual Studio).
Structure each project as a directory tree with a single root.
Create an automated build script for each project in its root directory that will build it from scratch, with NO dependencies on an IDE (but don't prevent it from being built in the IDE, if feasible).
Consider nAnt for .NET projects on Windows, or something similar based on your OS, target platform, etc.
Make every project build script reference its external (3rd-party) dependencies from a single local shared "library" directory, with every such binary FULLY identified by version: %DirLibraryRoot%\ComponentA-1.2.3.4.dll, %DirLibraryRoot%\ComponentB-5.6.7.8.dll.
Make every project build script publish the primary deliverable to a single local shared "output" directory: %DirOutputRoot%\ProjectA-9.10.11.12.dll, %DirOutputRoot%\ProjectB-13.14.15.16.exe.
Make every project build script reference its dependencies via configurable and fully-versioned absolute paths (see above) in the "library" and "output" directories, AND NO WHERE ELSE.
NEVER let a project directly reference another project or any of its contents--only allow references to the primary deliverables in the "output" directory (see above).
Make every project build script reference its required build tools by a configurable and fully-versioned absolute path: %DirToolRoot%\ToolA\1.2.3.4, %DirToolRoot%\ToolB\5.6.7.8.
Make every project build script reference source content by an absolute path relative to the project root directory: ${project.base.dir}/src, ${project.base.dir}/tst (syntax varies by build tool).
ALWAYS require a project build script to reference EVERY file or directory via an absolute, configurable path (rooted at a directory specified by a configurable variable): ${project.base.dir}/some/dirs or ${env.Variable}/other/dir.
NEVER allow a project build script to reference ANYTHING with a relative path like .\some\dirs\here or ..\some\more\dirs, ALWAYS use absolute paths.
NEVER allow a project build script to reference ANYTHING using an absolute path that does not have a configurable root directory, like C:\some\dirs\here or \\server\share\more\stuff\there.
For each configurable root directory referenced by a project build script, define an environment variable that will be used for those references.
Attempt to minimize the number of environment variables you must create to configure each machine.
On each machine, create a shell script that defines the necessary environment variables, which is specific to THAT machine (and possibly specific to that user, if relevant).
Do NOT put the machine-specific configuration shell script into source control; instead, for each project, commit a copy of the script in the project root directory as a template.
REQUIRE each project build script to check each of its environment variables, and abort with a meaningful message if they are not defined.
REQUIRE each project build script to check each of its dependent build tool executables, external library files, and dependent project deliverable files, and abort with a meaningful message if those files do not exist.
RESIST the temptation to commit ANY generated files into source control--no project deliverables, no generated source, no generated docs, etc.
If you use an IDE, generate whatever project control files you can, and don't commit them to source control (this includes Visual Studio project files).
Establish a server with an official copy of all external libraries and tools, to be copied/installed on developer workstations and build machines. Back it up, along with your source control repository.
Establish a continuous integration server (build machine) with NO development tools whatsoever.
Consider a tool for managing your external libraries and deliverables, such as Ivy (used with Ant).
Do NOT use Maven--it will initially make you happy, and eventually make you cry.
Note that none of this is specific to Subversion, and most of it is generic to projects targeted to any OS, hardware, platform, language, etc. I did use a bit of OS- and tool-specific syntax, but only for illustration--I trust that you will translate to your OS or tool of choice.
Additional note regarding Visual Studio solutions: don't put them in source control! With this approach, you don't need them at all or you can generate them (just like the Visual Studio project files). However, I find it best to leave the solution files to individual developers to create/use as they see fit (but not checked in to source control). I keep a Rob.sln file on my workstation from which I reference my current project(s). Since my projects all stand-alone, I can add/remove projects at will (that means no project-based dependency references).
Please don't use Subversion externals (or similar in other tools), they are an anti-pattern and, therefore, unnecessary.
When you implement continuous integration, or even when you just want to automate the release process, create a script for it. Make a single shell script that: takes parameters of the project name (as listed in the repository) and tag name, creates a temporary directory within a configurable root directory, checks out the source for the given project name and tag name (by constructing the appropriate URL in the case of Subversion) to that temporary directory, performs a clean build that runs tests and packages the deliverable. This shell script should work on any project and should be checked into source control as part of your "build tools" project. Your continuous integration server can use this script as its foundation for building projects, or it might even provide it (but you still might want your own).
@VonC: You do NOT want to work at all times with "ant.jar" rather than "ant-a.b.c.d.jar" after you get burned when your build script breaks because you unknowingly ran it with an incompatible version of Ant. This is particularly common between Ant 1.6.5 and 1.7.0. Generalizing, you ALWAYS want to know what specific version of EVERY component is being used, including your platform (Java A.B.C.D) and your build tool (Ant E.F.G.H). Otherwise, you will eventually encounter a bug and your first BIG problem will be tracking down what versions of your various components are involved. It is simply better to solve that problem up front.