Well this is bad negotiation for a lot of reasons. I've coached 1000's of folks on getting better offers. You are right that you have to have some leverage. But walking away is only one of many possible leverage points. You could also have other offers, have done well in the interview process, be walking away from an existing role that offers xyz that isnt offered at this company... What data points or pieces of leverage do you have other than I will walk? Because that is and should be the last piece used.
If you are putting the first number out you're already working from behind.
Company: We'll offer you 100K.
Candidate: I'm really excited about the role for xyz reasons and I liked meeting with (insert persons name here) because it allowed me to learn about (insert reason you actually like the company here).
I had a question about the offer. The base salary was a bit lower than I was expecting. Do you think we could get it adjusted? (If you have a point of leverage insert that here like, "My current comp is actually pretty close to this offer and I am expecting an promotion sometime in the next 6 months based on conversations with my manager").
I hope we can work this out over the next few days because I think this opportunity is the right next step in my career.
Let me know what you think. Thanks!
Company: "Okay we can do 115K"
Candidate: Thanks so much for getting those adjustments. I was targeting closer to a 150K base salary. I am very invested in the role and the company though. I'm specifically excited about working with XYZ.
Do you think we can move the base salary closer to that 150K mark? If not could we get a sign on bonus for the difference?
If you could get to 150K base salary or something like 130K with a 20K sign on bonus I'd be ready to sign today and could start 2 weeks from Friday.
Company: probably comes back with something less than 150K but something a whole lot better than 100K.
I've had exactly one person I coached ever have their offer pulled. He basically said here are all the reasons your company sucks and I need XXXk to come work there.
That's not negotiating. Negotiating is finding a point where you and the company both feel like you are both fairly valued.