Jeremy Keith writes about the challenges of turning large, established desktop-centric sites into responsive sites in Climbing Mount Responsive. This method remains my favorite:

Rebuild the mobile site, using it as a seed from which to grow a new responsive site. On the face of it, having a separate mobile subdomain might seem like a millstone around your neck if your trying to push for a responsive design. In practice though, it can be enormously useful. Mostly it’s a political issue: whereas ripping out the desktop site and starting from scratch is a huge task that would require everyone’s buy-in, nobody gives a shit about the mobile subdomain. Both the BBC news team and The Guardian are having great success with this approach, building mobile-first responsive sites bit-by-bit on the m. subdomain, with the plan to one day flip the switch and make the subdomain the main site.

I also really like Brad Frost’s illustrations of this approach in Planting the Seed for a Responsive Future.