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!
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.