How do I safely pass objects, especially STL objects, to and from a DLL?

How do I pass class objects, especially STL objects, to and from a C++ DLL? My app has to interact with 3rd-party plugins in the form of DLLs, and I can't control what compiler these plugins are built with. I'm aware that there's no guaranteed ABI for STL objects, and I'm concerned about causing instability in my app.