Do it before you announce it. Announcements give you the reward before the work.

Telling people about a plan delivers a portion of the satisfaction in advance, and the mind does not distinguish sharply between applause for the intention and reward for the work. Having already collected part of the payment, you are less driven to finish. Silence keeps the account open.

Keep the next project to yourself until there is something to show. Tell one person if you need accountability, and skip the wider announcement. Let the result be the first anyone hears of it.