Here's something I've never seen anybody do, but I imagine it would work: sell refusal to support IE6 not as a technical decision but as a marketing one - a mark of exclusivity.
Consider this solid-gold iPhone case:
It sells for $100,000. A cheap plastic case probably does a better job of protecting your iPhone. But a cheap plastic case does a much worse job of broadcasting your wealth to everybody around you, which is what this iPhone case is for.
Telling regular people to upgrade their browser is like saying they need to manually modify what they consider to be the dangerous, mystical internals of their computer. Most people don't upgrade, they just buy new machines. But if you say, look, your computer has to be at least this new to use our web site, you're speaking in a language of money, class, and social status, which is a language everybody understands.
I'm absolutely not saying this is the way things should be, but anything which gets web developers out of supporting Internet Explorer is worth a shot.