Following my call in December to revive Victorian slang, I think we should do much the same with 1920s slang too. All in good time, though - first please bear with me while I go and get as boiled as an owl on coffin varnish.
Just as words and terms emerge, gain currency and then drop out of common usage over time, the same is true of grammatical features. Here David Marsh, author of For Who The Bell Tolls: One Man's Quest For Grammatical Perfection, identifies ten rules that he argues should be disregarded, as well as five things with which people should take more care. (Or should that be "five things that people should take more care with"?)
(Thanks to Abbie and Simon for the links.)