Salary negotiation doesn’t begin when an offer is made. It starts at the VERY FIRST MOMENT you make contact with a company, and is continuously being adjusted and re-framed throughout the hiring process.
Never start negotiating a job offer when it’s presented. Get as many details as you can (you don’t want to have to place a follow-up call before your counteroffer), say thank you, and ask for a day or two to consider.
How to start a salary counteroffer:
“Everything about the offer’s great. The salary mentioned is ok, but [based on my experience and how it ties into the position] I was hoping for something closer to [SPECIFIC dollar amount].”
Then wait for a response! Avoid the temptation to over-explain.