It is a conundrum for many companies. I deal with it a lot and cannot stress enough the importance of ensuring that the vision for the brand is established prior to development of artwork, collateral and websites etc. Ideally a single person from within the business and one from a branding/creative agency should remain the custodians for the brand to ensure everything remains on track. If brand development is approached in sections and guidelines are established, then the results will be well thought out and unified. Too many times parts of the brand get farmed out to the cheapest supplier and then the result is a mess.