Let's start with Jeremy Bullmore's insight on insights - why is a good insight like a refrigerator?

The IPA has a mini library of creative briefs from different agencies for you to compare-and-contrast.

A little history of briefs and some models that should work too.

A tip: Don't obsess about the creative brief to the detriment of the creative briefing. Sad to say, but I've worked with planners who spent days and days agonising over the document, and then more or less dropped it on a creative's desk and ran away. I've always found that chatting to the creatives before you even out pen to paper is incredibly useful -this engagement is the start of the briefing process. And when it comes time to formally brief the teams, don't just turn up with a copy of your agencies template. Bring some flavour along to inspire them -maybe some movie clips or a storybook or a song...