Here's what I've been doing to get the light right. I too use a black background. I'm using a Nikon D200, USB cable, Tripod, Nikon SB-900 Speedlight flash and a program called Camera Control Pro 2.
You can set everything with the PC (Exposure, Focus, Shutter Speed, Aperture, Exposure Comp. plus everything else the camera does and because you're using the PC to shoot, no camera vibration.
Now the weird part. I set the shutter speed for 5 seconds, click "Shoot" on the PC and use the flash by hand with 2, 3 or 4 flashes around the coin to get an even exposure. The shot shows up on the PC in about 3 seconds. If it's not what I want, I delete it and try again.
I know this sound like a pain, but you get great shots.
You don't need all this equipment, but try a long exposure and a hand held flash from different angles.
Here's an example: 5 second exposure , f/36, 4 flashes - 2 from each side.
