1.  A decent cart operator.

2.  A real tractor on the cart, so the operator can see over.  

2 can be done without if you have 1 and at least a small window to look into the cart through.  

I'm definately in the camp which gets annoyed when I have to worry about where the idiot in the cart is going when he's supposed to be staying under my auger.  I've run a cart plenty and I KNOW it is not a problem to load the damn thing without being a bother to the combine operator....if you are doing a good job, you'll have to tell him when he's empty or you are full because he won't be watching....he's got other things to do and he doesn't have to.