Keep good records. Make a copy of everything you send or deliver to anyone. If follow-up is required, send copies with your inquiry. Always know your correspondent. Make a written record of all telephone conversations.
Memory is worse than invisible ink.
Make notes and reduce them to a narrative statement of your understanding of the conversation. Include an agenda of who is to do what by when. Obtain a receipt for hand-delivered material. If you are facing a deadline and a refusal to sign your prepared receipt, give the material in exchange for the recipient's name. Then, send a letter confirming that person's receipt of the material.