I do not care if someone gets in front of me. Traffic is not a line. That person moving in front of you doesn't stop you from getting to your destination. As long as do not have to stop, you will get there.
With that type of attitude, it's really easy. I make a three second space between me and the car in front of me. This allows another to easily move over when/if needed. At that time, I simply gradually slow down until that three second space is back. Often, I don't need to, because the vehicle that moved over in front of me just wanted the exit. By giving them the space to reach that exit, I prevented them from having to cut someone off and the resulting slam of brakes for 20+ cars back.
It's really easy once you remove the mindset that the road is a line and anyone in front of you will slow down your movement.
Your movement only stops when people have to cut into too small a gap between vehicles.