1. Start out on flat roads until you can stay on your board. Then learn how to push with at least one foot. Pushing just means keeping one foot on the board and pushing off with the other one.
2. Experiment with pumping/slaloming. It's a way of gyrating your body to keep you and the longboard moving. This guy shows several different techniques and is kind of intense but you get the idea. Basically start trying to shift your weight around until you find that you're moving forward a bit longer. I think it actually works better on smaller boards, but it's something fun to try while you're still learning your balance and pushing.
3. Find a helmet before attempting to go downhill. Start out with the slightest of downhills, and try carving. Carving and pumping look similar; in both, the board is travelling in an S shape because you're shifting your weight around. However, carving is meant to slow you down. It usually covers a wider area laterally than pumping (see this video around minute 2 to see a good example). This is why it is a stupid idea to try to longboard down a sidewalk--it's not wide enough for you to properly slow yourself down by carving. Make sure you understand this principle before attempting to go down steeper hills.
4. After you've managed not to fall off on a slight incline, look for a steeper hill. Then start at the bottom of the hill. Basically, you want to be able to practice controlling the longboard at gradually higher and higher speeds. If you start at the top or middle of a steep hill, it's likely that you'll lose control of your board.
5. If you need an emergency stop, jump off your board. You are more important than your board. If you are going very fast you will have to run to slow yourself down to a stop (after you jump off the board). Hopefully you can find a spot where a car won't destroy your board if you have to let it fly for several hundred feet.If you're keeping balanced on your board and your board starts fishtailing, slow it down by carving (and you might need to buy a better board or adjust your current one). I haven't learned to slide-stop yet but this is obviously a skill every longboarder should learn.
While I was writing this I found about.com's guide, which I think is new. It's kind of hard to find how-to resources for longboarding because most people learn from other longboarders in person. I also found this article that explains why skateboarders hate longboarders. Basically, longboards are a poor substitute for bikes in urban traffic and make skateboarders look bad. For long distances, bikes are hard to beat. But biking on roads isn't as thrilling as longboarding down a hill. And just cruising on a longboard is fine, I think, as long as riders can stop themselves.
Do you have any other good longboarding tips? I was experimenting with going down a steeper hill today and it was thrilling. I didn't fall, but I had a few close calls (and don't worry Mom, I was wearing knee pads).