One of the ways I like to get out of guard into cross body is to bait them to attack me with a triangle. I should clarrify that this has on occasion not worked out the way I initially planned but mostly it works.
I guess it's kind of hard to explain actually without it sounding too easy but I'll try anyway. Basically I move my hips 'around' so that I'm sort of walking my hips towards them in an arc and also give a little shrug. I have a really thick torso and wide shoulders on top of that so this is sort of a natural defence for me. I'm basically a big wedge when in someones guard but when I have shown this to other people it has tended to work well for them. It probubly has an official cool sounding name but I call it 'shucking' cus I'm shucking thier attack off and winding up in cross body.
one good thing to focus on is stopping them from starting the triangle, posture is good but it isn't the only way to hinder a triangle, the both arms rule is also good (both arms in or both arms out but not one and one)