A good tagline is like much most marketing. Lots of experiments with split testing of multiple taglines to see which one the market responds to the most.
As for the comments regarding branding, branding should be always done in-house without an agency. There's a misconception that outsourcing marketing is a good idea but often, the outsourced company doesn't know your business as well as you do. Read the Marketing Game by Eric Shulz(marketing for Coca Cola and Disney). He has some clear opinions about outside agencies. http://www.ericschulz.com/
As for the comments regarding branding, branding should be always done in-house without an agency. There's a misconception that outsourcing marketing is a good idea but often, the outsourced company doesn't know your business as well as you do. Read the Marketing Game by Eric Shulz(marketing for Coca Cola and Disney). He has some clear opinions about outside agencies. http://www.ericschulz.com/