Bench-mounted vises are nice, but somewhat limited in availability, and hard to improvise.
Most stubborn (so far) BB I've encountered finally yielded to a combination of a C-clamp locking the tool in place, a 5 foot cheater bar threaded over the wrench handle, and a helper sitting on the rack acting both as counterweight and keeping the bike upright.
I still pretty much had the rear wheel off the ground before the BB finally broke loose though.