Salt definitely works, but has its drawbacks. I'm sure the amount of salt used is important! One Xmas with loads of relo's over tried the salt method out. One esky, one case of beer(as emergency supply; had plenty in our two fridges!), enough party ice to cover the beer and about a third of one of those big Saxa salt plastic containers over the ice. Result- Still completely frozen the next day. In fact the salt had caused the ice cubes to form one solid block of ice. This was in the middle of Summer on the Far Nth Coast of NSW so the air temp was about 28. An excellent result I thought except for the drawbacks; you had to struggle to get a beer from the solid block of ice and if you forgot to rinse around the bottle-top with fresh water your first mouthful of beer was salty