What are the rules for leaving Thailand every 180 days?

So, every 90 days of continuous stay you need to report to immigration with your TM30. If you exit Thailand before 90 days and enter again then this clock resets.

Post first 180 days of continuous stay, you have a choice to extend your stay by visiting the immigration office.

Alternatively, you can exit Thailand at any given point less that 180 days of continuous stay by land or air and re-enter again.

Remember, you cannot make 180 days extensions twice in succession.