Know your audience.
Do your homework on your boss. Make sure you know their past and how he/she got to be where they are today. If you can understand your boss on a personal level that will be helpful in creating a connection. You don’t want to go in there and go straight to business. You don’t want your boss to feel that all you want from them is money. Show you care and are interested in them as a person. This way you can create a relationship built on trust and respect.
Ask WHY? “Why is it not negotiable?"
This will completely throw them off as they aren’t expecting it. It is likely you will be able to find out the real reason behind the denial. Ask for an explanation. There might be a very good reason. It also shows that you are not one to blindly accept what you are given. It takes courage to ask WHY. Most people would just say ok and accept it. Then, your follow-up question should be WHEN in the future could the salary be negotiated. Find out how often or not often this boss gives raises.
Forget your gender.
Most women self-sabotage by thinking that because they are women they will automatically be treated differently. That is not always the case. When you walk into your bosses office you are walking in as dedicated and strong employee - not a female employee. If you start labeling yourself by your gender it will absolutely alter you psychological mindset. It will put you at a disadvantage because you are associating that because you are a woman that your value is different. Forget your gender. Walk in with your head high. Have strong body language and good eye contact. And believe in yourself and why you have earned this raise. Your gender should have nothing to do with it, so don’t let that worry seep into your thoughts.
Maintain your professionalism.
I don’t care if your boss raises their voice. It gets tense. Or anything else. It will show how strong and confident you are. Never mimic any unflattering behavior. Nothing should deter you if you truly believe you have earned a better salary. Nor should you let anyone else's unprofessional behavior throw you off. Always carry yourself with grace and dignity, even if your boss doesn’t. You can still be assertive without being unprofessional.
Everything should always be face to face.
Negotiating your salary is extremely important. Don’t hide behind a computer or the phone. When you can look someone eye-to-eye it makes all the difference in the world. It also makes it harder for your boss to say no to you. People are always more brazen when they don’t have to address you in person. Make your boss say no to your face. And when he or she does, then ask WHY? Just as you are going in there and articulating why you have earned this raise, they should articulate to you why you haven’t earned it.