There are a lot of ways to negotiate salary and this tip is by no means the best one. However, in the right situation, it could be helpful. If you find yourself engaging directly with a firm (ie. not using a recruiter) and things are going well and moving quickly, stay with the momentum of the interview until you get all the way to the offer stage and then SLOW IT DOWN!

Here's an example:

You passively started looking around and either a friend, user group colleague or a former boss reaches out to you about an opportunity. You interview one time and you nail it. You come back the 2nd time and you meet with the CTO. This also goes well and since you're a known commodity and your experience speaks for itself, the CTO is ready to move quickly and make you an offer. Since everything is moving so quickly and going so well, a lot of times the software engineer will not negotiate as hard on salary as he should. After all, who wants to spoil the party? I have seen so many times where a software engineer has told me, "Well, I really did like the people and the work they were doing is cool so I didn't want to make a big deal over the salary". 

Well, if you think that way, you will leave a good chunk of money on the table over the course of your career. Most recently, I spoke with a really good developer. I don't want to call him "genius good" but he was damn good and a nice guy too. His salary he recently accepted was 90k. That same week I had another candidate take a job with a firm here in town for 105k. I would certainly put these two developers...