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.
In the even of a serious slip and fall, you should be sure to get a sample of any substance, if any, that contributed to the accident. You may have to preserve it in your freezer or refrigerator. In addition, store the shoes and clothes you were wearing in a safe place; you never know what the court may ask to see.