Here's a pretty clear tutorial video on how to slide and stop using one handed board grabs to slide and two handed push up slide to stop:
Chris McCurry and Jason Bither have a great collection of longboard how-to videos at SketeDeville - Their breaking section for how to stop a longboard is pretty solid:
If you've been experimenting with the longboard you've undoubtedly found out about 'speed wobbles' - TheGnarDeal and "Downhill/Dashboard Jesus" put some science to it:
Now that we have the nonsense out of the way, let's get back to business.
Endless Roads 2 - The Island with Longboard Girls Crew is is a bit slow out of the gate but phenomenal once the actually get rolling - around the 8:40 things pick up and by 10:45 mark things get wild! Smoking hot.
With huge fun and huge slides come some big crashes: