Try an enema using an enema bag. I find enemas essential during a fast and far preferable to a SWF. When done properly, a gentle cleansing enema can be virtually without discomfort, particularly when you are not constipated.
I've found that when I have a headache and take an enema, my headache is usually gone before I'm finished eliminating on the toilet.
Here are some links with more information on how to go about giving yourself an effective enema with minimal discomfort: