Convince the Boss
I do sometimes wonder whether we use the big bad client or the big bad boss as a crutch. “Oh, I’d love to try out this technique, but the client/boss would never go for it. Something something IE6.” Maybe we’re not giving them enough credit. Given the right argument, they might just listen to reason.
Go work on getting the right arguments for your specific client or boss, go cultivate a metric-ton of data to overwhelm whoever is holding back progress. No one is going to reach out and whack your boss or client with a "Web Standards Genius" stick for you, do the whacking yourself (figuratively, I do not endorse or condone violence against one's superiors, peers, or underlings). If after all that effort you're still not getting anywhere, then follow Mr. Keith's other piece of advice:
Another tactic that I’ve used in the past is to simply not ask for permission, but go ahead and use the new technologies and techniques anyway.