What happens if I forget to extend the registration of a name?
After your name expires, there is a 90-day grace period in which the owner can't edit the records but can still re-register the name. After the grace period, the name is released for registration by anyone with a temporary premium which decreases over a 21 days period. The released name continues to resolve your ETH address until the new owner overwrites it.
How do I receive an extension reminder?
Click the "Remind me" button on the name's page or your address page so that you can set a calendar reminder or email reminder. Note that you have to set calendar reminders per name, whereas you only need to set email reminders per the address of the owner. Also note that you can register a name for multiple years, removing the need to extend each year.
I lost access to the Ethereum account that owns a name I registered. Can I still extend its registration period?
Any Ethereum account can pay to extend the registration of any ENS name, though doing so from an account that's not the owner will not change ownership of the name. Just go to the name's page and click "Extend".