Whenever I use Remote Desktop to connect to an NT6+ (Windows Vista / Windows Server 2008 and later) machine, I use Network Level Authentication, meaning that authentication with the server is performed before session is created (contrary to first connecting to the server and using its GUI to enter the credentials). Usually this is a good behavior, saving me from man-in-the-middle attacks.
However, sometimes I wish to disable it at the client level, usually for troubleshooting.
Turns out it's not that easy. One can mandate NLA by using the Advanced tab, under Server Authentication:

but in order to avoid using it completely, you have to save your connection as an RDP file using "Save As":

And then edit the file using notepad and add the line:

enablecredsspsupport:i:0

Sources

http://technet.microsoft.com/en-us/library/ff393716%28v=ws.10%29.aspx
http://support.microsoft.com/kb/941641