Hi folks
If you ever come across a little book by Jean Aitchison
Language Change: progress or decay, Cambridge University Press (my edition 1991), you'll find all sorts of fascinating discussion on just these problems. Back in the 1960s and 70s a guy named Labov did a whole study on the way New Yorkers were changing the use of the "r" at the time. He did a lot of other work as well, with the sort of problems you've all been discussing.
Then there's the husband and wife team of Milroys, and they've studied (among other things) the way the men of Edinburgh were changing their pronunciation of "r" in a different way from the women - and for different reasons! (Lesley Milroy,
Observing and Analysing Natural Language, Basil Blackwell, mine 1987, pp114-7).
And Iain, you thought Lorikeet was mad!
Washing the cups is sending me a bit crazy, too. I'm almost agreeing with Lorikeet; but no: when I say "I washed the cups", the tip of my tongue moves forward to behind my (rather prominent!) front teeth, rather than remaining at the hard palate as in "I wash the cups". And there's also a slight change in rhythm.

But I'd say you be even crazier trying to teach it!
Cheers (In Oz: Chizz)
Norm