Another good training video is "Shotgun CQB featuring C.J. Caracci".
Chris Caracci is a former Navy SEAL. This is an excellent video that focuses on home defense shotgun fighting tactics and techniques. You can train with these methods in your own home using an unloaded shotgun or red gun.
"Shotgun CQB featuring C.J. Caracci" is included in the DVD "CQB Clearing Tactics for First Responders", which can be ordered from Amazon.com or rented from Netflix.
I don't use a sling on my HD shotgun (Remington 870P) because it's simply something else I have to manage to keep it from inadvertently snagging on objects while I'm moving.