If you think co-workers or others observed your wrongful treatment, make a list of their names, addresses, and home telephone numbers, along with a summary of what you expect them to say--whether good or bad. The bad or unfriendly witnesses are especially important to discuss with your attorney so that he or she can evaluate the damage they might do to your case. Forewarned is forearmed.
Ask friendly witnesses to give you a written statement of anything they saw or heard in person regarding your situation as soon as you decide to take action against your employer. Memories fade over time. Make sure the witnesses state only the facts of which they are personally aware and give specific examples of what they have seen themselves or what they were told directly. General statements such as, Everyone knew that the supervisor was out to get her, are not helpful to your case. Get statements that specify the who, what, when, and where of the retaliatory action. If possible, have the written statement signed in front of a Notary Public.
The most useful witness statements are fact-intensive and unembellished by anger toward your employer or by friendship to you. They should be detailed enough so that whoever reads them--the court, an attorney, or an agency investigator--will see the big picture. If you know of employees who were mistreated in the same way you were, ask them for statements about the way they were treated. If your supervisor, for example, made insulting and demeaning remarks to you and other workers, get statements from the other co-workers that quote or paraphrase the remarks, give the dates on which they were made, and name any others who were present.