For a more advanced, and much cooler looking way to stop, you can learn the Coleman slide. But, I recomend taking your time at first and getting comfortable just riding and cruising, before tackling the Coleman slide.
If you DO end up going too fast and getting out of control, then try jumping off your longboard! This sounds crazy, but it actually works very well. Try aiming your board at a lawn or something else that won't destroy it when you stop, and jump off the longboard.
To practice this, go to your large flat area, and push a lot and get a good speed going. Then, practice jumping off! You will of course want to land running. This might sound a little nuts, but it'll be good for you to know how fast you can do this. It's a good idea to not longboard faster than you can catch yourself running like this, until you get a little more skill under your belt.