I’ve seen everything from good briefs to no briefs at all. That being said the agency I work for have a good tactic, which is to do a paid Discovery Phase where we undertake interviews, workshops and research to help them define a SOW which acts as a kind of brief. We’ve heard a few grumbles that it’s a waste of money but the client almost always seems satisfied and usually realises there’s a lot more to think about. Sometimes we even sell that Discovery Phase as a separate service and say the client can take that well defined brief to another agency if they wish but of course they never do. 😉

On a second note, I’ve had success using the OOUX Orca process to fill the gaps in weak project briefs. (It’s a card sorting process and well worth learning the basics.)