While you can to an extent cover yourself with a "blood chit" signed by the passenger acknowledging that he understands the risks and will not hold you responsible it is impossible in law to cover yourself for negligence.
This is correct; I don't believe that any document, no matter how carefully drafted, can ever be regarded as a completely impenetrable sheild against all civil liability.
That said, even a poorly-worded document provides some argument that the passenger has voluntarily accepted the risk, which may deter some claims and would provide leverage for settlement negotiations in others. So, it's not a bad idea to require all pax to sign one, which should then be kept in a safe place (on the ground, not in the aicrcraft).
the damn lawyers
These sort of insulting comments are offensive and needlessly provocative.