In most cases you don't go down them. Bombing hills is a huge part of longboarding, but that's not something you can really do on a typical commute with vehicle and pedestrian traffic.
I haven't been able to figure out how to control my speed on a hill on a sidewalk (not wide enough for turns, and I can't really powerslide yet).
I saw an invention once which was a foot-operated brake for the back wheels of a long board. I think it had tiny drum brakes for the wheels or something like that. It had a big metal heat soak to absorb the heat from braking. Given I've never seen one, I guess it flopped.
There are various basic braking techniques like dragging your foot or reverse pushing. More skilled riders will periodically slide the board sideways to create friction and control speed.
