Implied Contracts
These days, implied and oral contracts are generally not valid legal theories against employers. If there is any language in an employee manual, or personnel document the employee signed, stating the employee is at-will or there is not a contract of employment there will almost certainly not be an implied or oral contract.
Even if you worked for a small employer without any written policies, you will have to affirmatively show that something happened worthy of the court implying a contract. Implied contracts can exist if the employee is a very long-term employee, or promises of longevity such as retirement are made.
Promises creating an implied contract may also include employees who are promised large sums of money or longevity if they institute a new process, set up a new location, or do something quite large for a company. However, any written language about the employer being at-will most likely will defeat an implied contract. Moreover, in the case of commissions, any written commission agreement is likely to defeat an implied or oral contract for commissions.