I'm knitting scarves and every few rows or so I notice that the working yarn is getting twisted between the needles and the skein, so I gently pull along the working yarn toward the skein to push the twists away. Eventually it gets too twisted so I hold the ball overhead and turn it in the direction opposite of the twists until it is completely smooth.

I'd bet this is a common problem, but how do I prevent this from happening? What am I doing to cause it?