Make sure to use plenty of cutting oil while you're doing it too.
I have had a local shop do a couple for me and it's pretty quick and cheap if they have done it before! And like everyone is saying... Make sure you thread a nut onto the fork before you cut it to your desired length! It will help "rethread" the cut area.
You might also double check ALL of your measurements BEFORE you cut!! Make sure you take into account ALL of the length you are going to NEED for upper truss rod brackets, light brackets, etc...