Communicate in Writing

When you go to court, a "he said she said" argument is always a roll of the dice.  Always communicate in writing.  If there is a conversation, put the contents in writing after the fact and send it to your ex-spouse.  Even if they later disagree with the contents of the writing, you will be able to point out what your understanding was at the time of the writing.  That is better evidence than "he said she said."