Courts will no longer find implied contracts based solely on an employees' long service to the company, or history of raises, promotions, or positive performance reviews. It can consist of a two sentence agreement that is distributed to and acknowledged by everyone. This protects you from claims that later words or actions changed the agreement. Make sure supervisors understand this. If you have an employee handbook, review it for consistency.
Offer letters are the most effective approach.
Distribution is key. Provide a copy of the acknowledgment to every single employee. A person who doesn't receive one can later argue that they aren't bound by it. For example, a provision requiring a progressive discipline policy may be interpreted as a contract requiring the employer to use that policy fully before terminating any employee. Employee handbooks can be a source of contract liability where employers fail to follow their own policies.