If you get going too fast not only does your danger level build, but you may experience the dreaded SPEED WOBBLES!!! (Dun dun duuuun). At certain high speeds and with certain board set ups, your board will start to uncontrollably pitch from left to right and eventually throw you off. Take this poor sad man for example. He had a bad case of the speed wobbles, and the lucky guy was stupid enough NOT to wear a helmet. Bad decision every time! He's lucky he missed the car and made it to the grass.
I lay there bleeding, groaning and wondering why I ever did something so dumb. My little brother came racing by me on his board, "Mason! Are you okaaaaaaaaaaaaaaaaaaaaaaaaaayyyyy!!!!!!!!!!" And he shot right by me as I lay there in the road. I eventually pulled myself off the curb and onto the grass and waited for my brother to come save me. Five or ten minutes later he returned and was somehow surprised, after my ragdoll asphalt acrobatics, that I was still laying down. He took me home and I was fine except for some roadburn and bruises. I am lucky beyond all reason that I didn't hit my head. When I flipped I really should have slapped my forehead on the ground. I got a lucky chance, and I've always worn a helmet ever since.
I can tell you from experience, check your speed, stay in control. Going fast is fun, you do look cool, until you hit a rock and go flying, or get speed-wobble-thrown and get all cut up. Controlling your speed is one of the first things you should learn.
"Mason, hold on a second," you may ask me, "there are no brakes on a longboard. How on earth can you control your speed. "Good question," I respond, "there are a few ways. Lets talk about them."
Ride It Out or Bail
If you feel confident enough you can just have faith and keep riding until your board slows down. If you're on a hill that levels out at the end you may be able to hold fast and ride it out. If you feel uncomfortable my advice is to bail before you get going too fast. Jump onto some grass. The sooner you get off, the slower you'll be going, and the risk of injury is minimized.
If your board has a kicktail you can push it down to the ground and use that as a brake. I would suggest getting a tailguard if you prefer this method or else the road is going to eat up your board. To be honest I would not suggest this method of slowing down. Lifting your two front wheels off the ground while going at high speeds is not the brightest or safest idea.
A fun but more expensive way to slow down is to slide. Basically your board goes perpendicular to your direction of motion and your wheels slide sideways on the road and you slow down. Its fun, and it looks really cool. I suggest looking up some videos on how to do this. It can get a little pricey because it causes damage to your wheels, causing flat spots that bump as you ride, and sliding-gloves are recommended as well. This can add up. This video below shows some pretty cool slides as well as some impressive footwork.
Turning your board from one side to the other, or carving, will slow you down. The harder you carve the more you decelerate (my physics teacher would hate that I just used that word). This is a lot of fun for riding and a preferred method of braking. Carve those hills! My one word of advice is not to play "Chicken" with the curb because you lose every time, the curb won't move out of the way, and Bonnie Tyler will not sing about her hero.
If you are going at a speed of 25 or more miles per hour you can stand up and spread your arms, pull your jacket out wide, just do everything to create as much wind resistance as possible. Certain companies sell "sails" you can buy, the problem is that wind conditions have to be near perfect, and one crash will tear up the "sail" material. Windbraking won't stop you but it may slow you down just a bit. There are still better methods however.
Brakes on Your Board
Some companies do make brakes for your board. I'm not sure how well they work but they look cool. You can check them out here Skatebrake.com and Brakeboard.com.
Besides carving this is my preferred method of braking. Basically you are dragging your foot on the ground and this slows you down. This can bring you from high speeds to a standstill if you want. You need good balance because what you do is take your back foot (or whichever one feels most comfortable for you) and you drag it on the ground. Start by putting the front pad and toes on the ground and slowly lower your heel down and try to apply pressure using the whole surface of your sole. As you can see from my earlier post on shoes, this does eat up the sole of your shoe. However, I think its better to have asphalt damage to your shoe than your face. This is why I recommend going with a cheap pair of shoes, a pair you bought specifically to trash. Practice foot braking because, in my opinion, it is the cheapest and best way to slow down.
Kahuna Big Stick
Now that you know all of these methods, get out and practice them. They will make you a more dynamic rider, and really increase your fun when you aren't falling and getting injured all the time. Most important though, WEAR A HELMET!!! Safe speeds and safe riding is no substitute for a good helmet. You can go brain dead moving at five mph or fifty. Wearing a helmet is your best tool to minimize injury. My personal favorite are Triple 8 helmets, and I'll explain why later, but just get some sort of lid on your head.