Dan, what do you mean by collapsing? Do you mean losing your arch and having your upper back take the load rather than your legs?
That won't happen so long as you lift with your chest first instead of your hips. Always keep your chest up and out, and make sure your hips don't rise too fast because that's what will cause you to good-morning the weight up, but I'm sure you know that already.
If you mean your upper back just feeling very loose, just make sure to really squeeze your shoulder blades together as tight as you possibly can, and retain that tightness throughout the movement.