Don't do it. WPF Windows are not designed like that, thanks goodness. You need to start with thinking about nice UI design.
There are good alternatives. One very simple but quite good one is
There are many others. Basically, you can have multiple panels in your window. Hide/Show them, move in/out, minimize/maximize, etc.
You can even use the advanced docking interface line in Visual Studio: http://avalondock.codeplex.com/