I was wondering if there is a way to pass parameters between viewmodels without using the ShowViewModel(MyParams). There cases were I navigate from FirstViewModel to SecondViewModel and back but I do not want to reinstantiate the viewmodels and loose my current property values. I dont know if that makes sense to you.
Use a Messenger class to send messages between View Models ...
You can have a look at some of the frame works in order to see how to implement them, or just use of the MVVM frameworks out there (I've used and am quite happy with MVVM-Light for example, but there are plenty out there).