Just expanding on what Mike said, make sure that the black cloth is fairly wide and high, because if it isn't, you'll get overshoot of the image and see the edges of the cloth.
It is a fine line to get the correct distance from the backdrop.
The other thing to help is to stand aways back and use the camera's telephoto lens to crop the flower in the view finder / screen and this helps eliminate image overshoot as well.
Have a look on my Web site:
http://www.users.on.net/~op_ink/AD/Welcome.html and go to Dendrobium and look at the difference between images 14 & 16.
Image 14 was taken utilising the telephoto and image 16 was taken just using closeup. You will see that image 16 shows more of the background than image 14.
Hope this helps.
