WindowsPhone 8.0:
In WP8.0,you can easily hide the soft keyboard by focusing the other UI element.
//To hide the soft keyboard,focus the current page
this.Focus();
WindowsPhone 8.1:
In WP8.1,Fortunately we have InputPane class which is available from 'Windows.UI.ViewManagement'.So InputPane class having two methods 'TryHide' & 'TryShow' for hiding as well as showing keyboard.
//To hide keyboard
InputPane.GetForCurrentView().TryHide();
//To show keyboard
InputPane.GetForCurrentView().TryShow();