Portable Class Libraries help here. They aren't perfect, and honestly they're justly badly reinventing Java's loosely coupled jars, but they give you the ability to link stuff together in a little more general of a way.
In my current project (a video game), the core library is a PCL - it contains data structures and the interfaces and abstract classes that content (mods as well as the base game) must fulfill. I have a "desktop" library that's a normal .NET 4.5 class library that contains stuff like "load your config data from files, include the runtime-compilation mod system", and I have runner projects--text-mode via curses for now, eventually graphical post-prototype--for MacOS, Windows, and Linux[1]. I use configurations (not platforms, leave everything as AnyCPU unless you want to hate your life) to provide preprocessor flags for #if; my solution has ended up with an ugly number of "Windows-Debug", "MacOS-Release", etc., but the differences between XS and VS make it hard to do things "right".
[1] - all source/content files added via a single template .csproj, using a Ruby watcher I'm going to eventually open-source, because using MSBuild correctly to handle this situation breaks in both VS and Xamarin. (That last bit, you may be realizing, is a common theme. .NET has such good stuff, but half of it is broken at any time.)
Assuming you don't need to pass around binary libraries, take a look at shared projects-- added in Visual Studio 2013 and Xamarin Studio 5, IIRC. Unlike PCL projects, they compile in the context of the projects that reference them, so you have full access to the libraries that are common between each platform, and (if your coding style allows it) full access to platform-dependent APIs as long as they're protected by the appropriate #if __IOS__/#if __WIN32__/etc.
Keeping common code in shared projects is like night and day compared to the mess that is keeping common code in a PCL and having to deal with common APIs that aren't present (or worse, are incomplete) in your chosen PCL subset. They really clean things up a lot, even if you're treating them like they're a PCL and avoiding any platform-dependent code.
Ooh. I didn't know about this. I'm using VS2012, because I don't want to shell out again for VS2013 but want ReSharper - I'll have to look at this...thank you!
In my current project (a video game), the core library is a PCL - it contains data structures and the interfaces and abstract classes that content (mods as well as the base game) must fulfill. I have a "desktop" library that's a normal .NET 4.5 class library that contains stuff like "load your config data from files, include the runtime-compilation mod system", and I have runner projects--text-mode via curses for now, eventually graphical post-prototype--for MacOS, Windows, and Linux[1]. I use configurations (not platforms, leave everything as AnyCPU unless you want to hate your life) to provide preprocessor flags for #if; my solution has ended up with an ugly number of "Windows-Debug", "MacOS-Release", etc., but the differences between XS and VS make it hard to do things "right".
[1] - all source/content files added via a single template .csproj, using a Ruby watcher I'm going to eventually open-source, because using MSBuild correctly to handle this situation breaks in both VS and Xamarin. (That last bit, you may be realizing, is a common theme. .NET has such good stuff, but half of it is broken at any time.)