Tires help but if your going down your going down. You need to be standing up with your weight way forward as far as you can go while the bike is still straight. Elbows bent and out on your toes and all that stuff. Then when the bike moves around you counter the movement sometimes very fast while staying on the gas. Basically the bike could almost be laying on the ground but you would still be standing over it.
When you need traction and you have the bike straight and pointed in the right direction and straight you weight the back tire.
All this doesnt work so well on a 600lb bike with heavy steering unless you are very strong but the heavy bike is actually an advantage unless the ground is real soft then you can sink if your speed is not sufficient to stay on top.
The heavier bike will actually move more slowly which helps if your technique is perfect but you can't save yourself by putting a foot down