4. Be ready with backup.
You are more likely to be convincing and come out on top in a disagreement when you have backup information. For example, if you believe that you are entitled to a benefit under company policy, bring a copy of that policy with you to HR. If you believe that someone's approach may violate a policy, present that policy with your argument. The next time you want to make your case, try preparing some backup information before your meeting. Follow rules one through three, then present your argument with the backup information you prepared. You may be pleasantly surprised at how persuasive you can be.
5. Some battles cannot be fought alone.
There are some disagreements that you alone cannot win. For example, if you feel that your employer is discriminating against you because of your sex, or doing something otherwise illegal, you will probably not change your employer's mind by pointing it out again and again. When you suspect that your boss is doing something illegal to you or otherwise violating the law, it is best to get legal advice or a lawyer to help you win your battle.