This is important because employers will sometimes ask your reference for someone else who is familiar with your performance. When checking, people may look to discuss your management style, ethics, work habits, people skills, liabilities, etc., in addition to confirming dates and incomes. Assume that employers will want to check your past superiors. Track these people down for at least the last three jobs or ten years. Don't be reluctant, even if you have not bothered to keep in touch.
People like to learn what is happening to others.
In the case of executives who have moved into top management, references from any but the last one or two positions are rarely needed. Let the employer know that you need to keep your activity confidential. This lets them know you have a worthwhile position to protect. If you have worked in only one job or for only one company for quite a long time, then contact former employees or bosses who have left your company and ask them to be references. If appropriate, consider using customers, suppliers or trade group contacts.