Website redesign

I have an established site that needs a rework. The site is looking a bit tired & I want to freshen it up & make the layout & interface more user friendly & 2012! This job is for the design & CSS code which we will replicate across the site. Database functionality etc is done by us in house so that is not part of the brief. Successful bidders need to show me that they can improve the site without alienating current users. Clean, simple design features are essential as is cross browser & mobile support.