[Stitch]

One way to break this is to, well, physically break it. I've read a number of places that crossed arms can mean a negotiation is working badly, but one way to break out of that pattern is simple: hand the person something, which breaks their "arm lock", which in turn breaks their internal closure; then you can do the usual mirroring, building rapport, etc. The analog to crossed legs is a little more complex, although physically moving to a new location is one option.