Clause Clinic
Confidentiality clauses tend to be divided into two: relating to confidentiality during employment and post-termination.
During Employment
Following the memorably named 1986 case Faccenda Chicken Ltd -v- Fowler, it is implied into all contracts that during employment an employee shall not reveal confidential information. Most employers will (and should) include an explicit confidential information clause drawing employees' attention to the duty not to reveal confidential information. Such a clause will usually state, in a non-exhaustive list, the kind of information that is confidential. It should be noted that information that is already in the public domain (save that which is in the public domain because of an employee breaching the confidentiality clause) cannot be made confidential by calling it so when defining the term.
Post-Termination
After employment, only trade secrets and equivalent highly confidential information may be protected by the implied term. Accordingly, for clarification and in an attempt to 'improve' the scope of what amounts to confidential information (however, see comments above) employers should use the express clause to specify what is regarded as highly confidential.
Post-termination confidentiality clauses work best hand-in-hand with other post-termination restrictions, such as those restraining competitive trade, and poaching or soliciting clients or employees.