There are books galore and guides on branding, but the most important element you must work out on your own is your point of differentiation. 

What is it that sets your company apart from its competitors that your best prospects want, need and are willing to pay for?  As a company ages and matures, it's essential to keep pace with often highly disparate changes in the marketplace itself. 

Rebranding efforts must examine what your company offers today, the nature of the target audience and what that audience wants to buy from you.  You may need to retool what and how your company markets or even revise the preferred target audience before you can effectively rebrand.