How to Stop
The quickest, and easiest, way to stop is to just jump off your skateboard. It keeps rolling, but you don't. Since you don't really want your board to just roll away, like into traffic where it gets run over you should only do this when you need to bail and it is too late to stop some other way. Afterall, it is better that your board gets crunched by a car without you being run over, too.
Foot dragging works, too. You just put your back foot down on the ground and drag it. Even if your going fast this will slow you down.
This the is probably the most common way to stop. You just put your weight on the tail and bend your front leg, the tail goes down and drags on the ground and brings you to a stop.
You can also use this if you're really moving fast by using the technique like a break. You just do it several times and slow down gradually instead of all at once.
If you 180 backside of frontside your tailstop you will stop even faster.
The most awesome looking way to stop a skateboard has got to be the powerslide. It is also the fastest way to stop a skateboard, too. To do this you turn you board to the side and skid to a stop, and you can do this even when your moving fast.
You need to be able to kickturn and be good at it before you try this, though, or it will probably be really painful.
Your feet will need to over the trucks. Shift your weight to your front foot and slide the back wheels around 90 degrees. The wheels have to stay in contact with the ground so that they will skid. This is different from a kickturn where one set of wheels is off the ground. Once the board is turned, lean back and push with your feet. The wheels should be skidding and you will come to a stop.
As you learn to do this you will probably end up doing some kickturns to keep your balance and you will probably crash a few times. Powerslides can be hard to learn but are worth learning. Pads aren't a bad idea when you learn this, either.