One helpful tip - if you're not good at it, try to avoid doing it in real-time. Negotiating via email is much easier for beginners.

In face-to-face communications, those not accustomed to negotiating will often give in just based on misread cues. For example, if someone asks how much you want, and you say a number, stop talking until you get a response. Many beginner negotiators will interpret silence as disagreement (even a short amount of silence) and quickly add "but that number is negotiable".

I have this happen all the time when I'm doing profiles of new candidates, particularly when they are junior. They say "I"m looking for 60K, but I am willing to negotiate". I usually respond by telling them they have already backtracked before receiving any objections.

Practice or get some coaching, and until that time, try to do it via email.