the half-million disclaimers used at YouTube is apparently "No Copyright Intended" which -- despite its ambiguous meaning -- is about as effective as going 90 MPH in your car with a sign that says "No Speeding Intended." Other disclaimers state that the user is claiming fair use. Unfortunately, claiming fair use doesn't mean that the work qualifies as a fair use. Only a court can determine that. In truth, we don't believe that there is really any disclaimer (scroll to bottom of page) that would be effective in preventing a lawsuit if your video infringes someone else's work.
So what do you say? Knowing that disclaimers won't prevent a lawsuit, you might be able to limit damages in a lawsuit by stating something to the effect of "No copyright is claimed in [content copied] and to the extent that material may appear to be infringed, I assert that such alleged infringement is permissible under fair use principles in U.S. copyright laws. If you believe material has been used in an unauthorized manner, please contact the poster." Don't expect this to do much -- most copyright owners will bypass this and have it removed under a DMCA notice -- but it may set a more sympathetic tone for you as a defendant if you find yourself responding to a lawsuit.