Lead with the answer
Not with an apology, not with a restatement of the question. The first sentence should say what will happen or what the customer should do. Everything else is supporting detail.
Leave deliberate gaps
A good template has one or two places the agent must fill in. If it can be sent untouched it will be, and customers can always tell — usually because it answers a question adjacent to the one they asked.
Placeholders for facts, not for feelings
Name, ticket number, order reference and dates should fill automatically. Empathy should not be a merge field; a templated apology reads worse than no apology.
Keep them short
Templates grow. Somebody adds a caveat, then a link, then a disclaimer, and within a year the reply to a simple question is four hundred words. Review quarterly and cut.
Review dates matter
An out-of-date canned reply is worse than none, because it is sent confidently and at scale. Put a review date on every template and honour it.