What's the best way to slow down on a snowy highway?

Brake early and often. Downshifting can be OK, but it can cause you to slide. Now pump is a relative term. The tire needs to be rolling to slow the car down. Slamming on the brakes, and locking the tires will induce a slide, which pumping wont help. Light brake pressure is your best option. ABS only activates when it senses a locked wheel skid, and by then you are probably too late in the snow.