When opening any new document in Illustrator, choose File > Document Color mode > CMYK. To Check this before submitting your file for upload – in illustrator versions CS4 and CS5, you can open your Output-Separations Preview window. Click on the drop down window showing Off and select Separations. Click off the black eye and everything left showing on your file is not in 100%K. If you select a part of your file that is left, and look at your color palette, anything that shows a percentage of C,M, or Y needs to be corrected. To correct this – go to Select > Same > Fill (or Stroke). It should highlight everything that looks like black (but isn’t). Than go to the color palette and using the eyedropper, click on the black part of the bar (far right). The C, M, and Y will go to 0% and the K should go to 100%. Now this part of the file should disappear because it is correctly set to 100%K.