When you reach an agreement on something, write it down and date it. You don’t need a long contract, just enough words to sufficiently describe the agreement. You can put limitations on it and agree to abide by it for a day, month, year, or more. Set parameters so under particular circumstances, the agreement is void.
You can also keep track of your agreements in a shared spreadsheet or document that lists all the agreements and the rules or terms of them. That way everyone involved has easy access to it to help them remember.
Sometimes, agreements are difficult to reach. In which case, you can make an agreement to disagree for a limited amount of time and then plan to sit down together to rehash the topic to see if you can agree then.
Parents can use this tool with their children. You write the agreement, that they will receive particular privileges if they perform certain chores around the house.