If you want to try steeper slopes and hills, that's great. Don't pick anything extreme, just something a little steeper than the slope you were trying on. Roads work well for this, just make sure there isn't going to be any traffic. Having a friend there to watch for cars is helpful!
And please, make sure you wear a helmet, even if you think you are good enough that you don't need to. One of my friends ripped down a huge hill on a longboard I was reviewing, lost control, and ended up in the hospital for a head injury - if he had been wearing a helmet, he would have been fine with only a few bumps and scratches. Instead, the guy couldn't skateboard for two weeks. If nothing else, you don't want to lose good riding time to a dumb injury - wear a helmet. And it wouldn't hurt to wear elbow pads as well, if you plan on getting up some good speed.
Longboarding down a hill is exactly like longboarding down a slope, you just go faster. Plus, stopping is a little more tricky. Don't be discouraged if you fall and get a little banged up - shake it off, and get back on your board.
That's basically all you need! Longboarding is an art, just like skateboarding, and the more you ride the more you will develop your personal longboarding style.
Photos for this article are courtesy of silverfishlongboarding.com