It's a good idea to keep a written pain and injury log after a slip and fall accident, along with photos of any visible injuries. If you don't, you'll likely to forget the extent of your suffering before the case makes it to court. Keep track of the relevant prescription meds, too.
Many large businesses keep accident report forms on hand, if only for their employees. If you're hurt in a slip and fall, ask the manager for an accident report form to fill out, and make copies for them and you. If there are no forms available, take copious notes of the conditions that may have contributed to the accident.