To trim overtime costs, we are considering paying the clerical staff on a salary basis. Do we have to get them to sign some kind of agreement?
An agreement will make no difference. If the employees are non-exempt, simply paying them a salary will not make them exempt employees. Even an agreement to not be paid overtime is not enough, since employees cannot waive their right to minimum wage or overtime pay. It is generally best to pay non-exempt employees such as clerical staff on an hourly basis. That way, if they ever run out of paid leave and miss work, the company does not have to worry about getting their written authorization to deduct the value of the unpaid leave from the salary.
