I have a single re-entry permit and will be leaving and returning to Thailand later this month. Best I can tell from reading other entries here is that when returning to Thailand I should enter the Re-entry Permit number in the Visa No. space on the Immigration Arrival Card. Is that right? Any other gotchas I should be aware of when leaving or returning to make sure my visa doesn't expire? Thanks for the help.

Yep. Enter the Permit number on the TM6 arrival card.

Make sure the Officer sees the permit and check the permission to stay date in your passport before leaving the Immigration area.