Disclaimers in an offer letter, employment agreement, and/or employee policy handbook can help employers avoid contractual liability toward at-will employees.
Disclaimers should provide that:
the employee handbook is not a contract
the employee handbook may not be modified except by certain specified procedures and by certain company officials
the employee handbook does not alter "employment at will" status - it is common for an "employment at will" disclaimer to appear at both the beginning and the end of an employee handbook