It is critical that you keep the file clean as noted above. I recently learned something interesting. The above noted File Card which is used to clean a file, is also good at dulling your files. A better solution is to use a block of wood pushed across the file teeth to dislodge chips.
I can attest to the damage done by trying to draw file with a clogged file. It can be heartbreaking to be amost done, and then put a big gouge in your work because you tried to make one more pass before cleaning the file.
jmx