As with anything it's about practice, negotiation is an art. For an unskilled negotiator the best thing to know is your BATNA.
A great rule of thumb is if people aren't saying no you didn't ask for enough.
The first time I negotiated I shot way over, apparently even the lawyer didn't make that much, my reply was that the lawyers work usually wasn't scalable across multiple clients and that any code I made was a capital good that could be resold again and again generating value in the company for years, then I pulled out a sales spreadsheet showing how the code I wrote for various product lines brought additional revenue to the company and then multiplied by the average length on the contract. (Make a really big number and ask for a small part of it).
Needless to say I was so successful in my negotiations that I was no longer allowed to have access to sales data.
The best part is once you negotiate a good salary you can use it in future negotiations as an indication of skill to get even better salaries. (eg. Does the competitor want to steal the $50K per year coder, or the $250K per year coder?)
For salary negotiation, I don't think knowing your BATNA is that helpful. Your BATNA is either accepting their initial offer or walking away.
I think your best course of action is to gather up salary data based on the job responsibilities of the job you're applying for. There are lots of online resources for researching salaries and you'll be able to grab data points like median, and quartiles. You can then massage that number up or down based on extra criteria like your unique skills, additional education, etc.
In any negotiation, finding objective criteria that you can show will always put you in a stronger position than creating semi-arbitrary bargaining positions.
The other best advice I could offer to would-be negotiators is to avoid setting any initial price until it's no longer reasonable to avoid it. If you have to name a price, name a price you're going to be very happy with.
My BATNA has rarely ever been accepting their offer, it's usually been already having an offer in hand, or having a bunch of interviews lined up in the coming days.
Objective criteria a horrible way to negotiate, look up the accounting term 'good will', you create 'good will' by not using objective criteria.
Name a price they will say no to, not one you're happy with, if they counter below your BATNA, tell them the mininum you'll accept is something above your BATNA, give them a timeframe to come correct, get up from the table and leave.
I could careless how badly the average, or 5th percentile employee is paid. If they want to retain me they'll a price better than my BATNA.
BATNA is best alternative to a negotiated agreement, meaning, it's what happens if you don't agree. So really, a BATNA in a salary negotiation for a new position is not taking the position. In an existing position where you're going for a raise, your BATNA is quitting.
BATNA isn't a bargaining position, it's your fallback option in case bargaining fails. I think you're talking more about your "bottom line", which is the lowest number you're willing to accept. A good negotiator goes in knowing his/her BATNA and bottom-line, but they're separate concepts.
I also completely disagree with you disregarding objective criteria. I used my recent purchase of a 2-year old vehicle as a way to gain some extra experience in negotiating and work on different approaches with two separate dealers. My initial approach with the dealer I ultimately bought from, was to derive my bottom-line number from objective criteria, but to show no strong evidence to how I got the numbers I was offering. I got nowhere with that approach. The sales manager didn't even come out to stop me from leaving when negotiations with the salesman got nowhere. We were only $1,000 from my bottom-line (which he didn't know) and about $1,800 away from our stated positions.
I came back a few days later with reasonable evidence based on market data of the particular model I was buying, as well as reasons why I'd go spend more money on another manufacturers' similar, but nicer model in the same class. They ended up coming down the full $1,800 after I was about to walk away again.
This works time and time again. Unless you're in a position where you completely don't care if you complete a purchase/sale/agreement, then holding onto unrealistic positions will never get you very far. My father-in-law likes to buy and sell stuff, but he pretty much only deals in unrealistic offers, and he rarely ends up buying or selling anything, even though he spends a decent amount of time looking.
If they are a strong no, try and trade off that $15k for increased benefits in some way
If they are a weak no, lower your ask.
They will almost certainly give you $5k above what they quote, maybe $10k
And keep in mind: if you're a typical software engineer going through a typical full-day on-site software engineering interview, they've dumped the better part of $1000 worth of dev-hours just to interview you (double that if they flew you out!) so seriously, asking for an extra $5k is no skin off their back. Just do it.
Read 'Getting to Yes' and 'Bargaining to Advantage' for more specifics about this discussion. I think everyone should read them. It doesn't hurt to include 'Influence.'
