Is there a way to prevent a message from accidentally being sent?

Type "zzz" (or something else that won't resolve to a valid email address) into the CC or BCC field. Outlook won't let you send the email until all the email addresses are resolved.

I create a client-only mail rule called "Defer Send".

1. Create a new rule, starting from a Blank Rule "Apply Rule on messages I send"
2. Have it process for all sent mail.
3. Select "defer delivery by a number of minutes"

Sometimes I'll add "except if it is marked as importance" as well.

This way, if you accidentally send, you can go and reopen it from your outbox, and it won't be sent.

While I would prefer to go with CharlieRB's solution any day, a quick way to achieve a similar effect is to simply go offline for a while:

Then go back online once you've finished writing your mail.