I reckon if you don't do negotiation on a regular basis, learning to keep your mouth shut, as this post says, is the single best and easiest thing you can do to improve your outcomes.
When you ask your question (this one is great btw - what can you do to make that number better), the other side may, subconsciously or (for more experienced negotiators) consciously just say nothing for a while. They're waiting for you to crack - to immediately fear the silence and blurt out something like "if that's possible of course" or "it would really help me, I've got 3 kids to feed". Following up your own ask with an immediate mini-backdown like this - without even making them say their piece - will lose you $.
Just wait. Silently. Lock eyes with them, not aggressively or weirdly, but patiently. After ten seconds (which may feel like 60), if the heat gets too much, raise an eyebrow slightly, clasp your hands, something.
Nothing's guaranteed but if you can do this you'll get better outcomes more often. And that's especially true on larger deals with very elastic prices (like selling an expensive piece of software) than something like your salary which often has real and hard bounds around it.