A proper diet is key to overall health in general...not just for FMS.
Due to multiple chemical sensitivities involving FMS there are some foods that can irritate the symptoms. The only sure way to tell this would be to delete a substance, see if one feels better, then add the substance back into the diet to see if one feels worse. If you suspect food allergies, testing can be done (but this is painful and can be quite lengthy).
Coffee, sugar, chocolates, artificial sweetners, dyes, food additives, gluten (and many more) can all effect FMS and induce symptoms. It is recommended to add more fruits and vegetables to the diet, as well as chicken and fish...using red meats sparingly.