There are several ways, but the one that is rage these days is MEF. This series of publications focuses on the composition of the Silverlight user interface, but the methods are essentially the same (except for packaging and deployment):
http://codebetter.com/glennblock/category/hellomef/
, , MEF, Pub/Sub Prism "EventAggregator", , EventAggregator ( ... , ), , , Messenger MVVM Foundation (http://mvvmfoundation.codeplex.com).
, MEF Prism, .