Familiarize yourself with the terrain before attempting an expert black diamond run. If you have never snowboarded the run before you should take into consideration the snow conditions and the reputation of the run before attempting it. Icy conditions and an abundance of trees make for a dangerous run on a black diamond.
Use your arms for balance. Once you have decided which run to tackle, stop worrying about looking cool and stick your arms out to your sides. You need this extra balance to maintain control until you get more experience on the black diamond runs. Once you have the feel for the run, you can work on cleaning up your form.
Control your speed to avoid losing control. This is best done by keeping your board perpendicular to the hill and never allowing it to point down the hill for very long. Ride across the run as much as possible to keep your speed to a manageable level. As you become more confident on the slope you can increase your speed and still ride safely.
Use the bumps to your advantage. Most black diamond runs will have moguls built up from hundreds of skiers' and snowboarders' carved turns. Either use the dips carved between the bumps as a trail to follow, or use the top or edge of the mogul to pivot on, exerting pressure to scrub off speed. Look ahead as you make your way down the mountain, plotting your path through the bumps as you go.