Write your boundary line: A simple, respectful decline you can repeat. e.g., I won't be able to take this on right now.
Write your backup line: A doable alternative you can offer. e.g., I can share a template / review one page / do it by Friday instead.
Use the pair: When a request misfits, say the boundary line once - then offer the backup if helpful.
Save both lines in your Notes and say them aloud once.
Offer only what you can truly deliver. Use written channels (email/chat) if live declines feel tense. Review your lines monthly and adjust wording to your voice.