I'd have to say first and foremost.... FLEXIBILITY! Yep, It all starts with your ankle flexibility. Think about it. No, really think about it. If you have ankles, hips, that are not flexible you tend to rock forward. I had similar issues when I was younger. Stretch stretch stretch your calves both with your legs straight and with your legs bent (Stretch the gastroc and soleus) get that ankle flexibility...Stretch the groin, hams, quads, hip flexors!
In addition one thing that will really help is drive your elbows down throughout the squat. Keep your elbows forward and down throughout the exercise. It forces a straight back. When you start to round your back take note of where your elbows are. They're high...
Lastly, your abs and erectors are analogous... Meaning they assist one another. (I'm sure you know all this) For every heavy GM's you do you shouldn't neglect your ab work. People with rounding issues tend to improve with a tight midsection. Keep it tight throughout.
Lou Ferrigno suggested that you actually squat without a belt (Obviously lighter lbs.) that really forces you to keep straight and improves tightness in your midsection or he also suggested to actually turn your belt around so the wide part is in the front. That was his opinion....