Enable Windows 8.1+ Per-Monitor DPI support for Desktop WPF Apps. Instead of attempting to understand this long MSDN article, take advantage of my personal suffering and use this instead:

Install-Package PerMonitorDpi

public MainWindow()
{
    new PerMonitorDpiBehavior(this);
}

To observe the difference, attach a normal monitor to a Surface Pro 2 or other Retina-DPI monitor, then move your window between the two monitors. Per-Monitor DPI apps will stay sharp, normal apps will have blurred text on the Retina monitor.

The Right Thing™ :) Older versions of Windows will use the system-wide DPI information instead.