I do not have a natural affinity for salary negotiations, because there's so many damn unknowns, and what if I insult someone and lose the offer, or they find someone better while they're waiting, and dude I already pay more in taxes than some people earn in a year, so why get worked up over it.
This is, of course, bullshit. Someone's going to be making money off of me, and it might as well be me. This a rare moment where agencies are actually add something to the economy. The agency is not smart enough to disguise it's attempt to push my salary down, the techniques it uses to attempt this piss me off and motivate me to fight back, and it doesn't hurt my relationship with the people I'll actually be working with.
That said, back-and-forth negotiation is an uphill struggle, because I'm trained to deal with facts and logic, and the person I'm talking to is trained to screw with words, and has had this conversation many more times than I have. When I had to negotiate a salary bump to compensate for a decrease in benefits, the recruiter talked circles around me. My defense was to simply repeat my points- "I delivered the work, you deliver the money. Benefits are money."- until she caved.* But I hate doing that, and I won't always have their balls in quite such a vice grip, so I'm always looking for simple tricks that simplify things.
The first is, as I mentioned, arguing from data. I happen to have an gold-plated starting point in the form of a well-paid full time offer. Even though I turned it down (and thus can't leverage one against the other), it's a marker of how much I'm worth. Agencies will always try to make you feel like you're asking for an unrealistic amount, but if you stick to data, it's hard for them to do so. I'm already preparing arguments for when they argue that my rates are unrealistic, and it involves making them send me data. Also, keep in mind that no matter how many times they insist the company is only willing to go to X, they are lying. The company is paying them X+20, minimum. What they're negotiating for is their profit margin. Since in an optimum world they wouldn't even exist, I don't think that margin should be high. I've never heard a first number for an hourly contracting rate that couldn't go hire (disclaimer: small sample size).
There's also pre-screening. The rate I listed on dice is really the upper bound of what I realistically expect to get, but that's okay. Recruiters will offer me jobs that pay less, the only jobs it scares off will be those that paid too low for me to consider. Lots of studies have shown that asking for a large amount increases both the money and respect you eventually receive, even if you don't get as much as you ask for. It also should bring me a more interesting class of job, since only the high-level jobs will pay anywhere near that much.
For that extra inch, ask for extra time, either between offer and decision, or decision and start time. I got $2/hour to start a week earlier, and a friend got $5/hour to accept an offer immediately.
Lastly, don't be afraid to walk away. You only need to find one company that's willing to pay your goal, and it's okay if the others can't, because you're not as good a fit or their product isn't profitable enough. I don't like the idea of outright rejecting a job as a negotiating tactic, but I'm not sure if that's actually sound policy. If you're turning down a job for reasons other than money**, accepting it for more money will just make you miserable. If you want only a bit more, outright rejection is too extreme. I would just ask for more money. The only time I'll use rejection as a tactic is when I think they're lowballing me, at which point I'm okay with saying "nope, not enough money, but thanks." This is true even if I don't think they're taking too large a cut of what the originating company is offering them- my skills are more valuable to some companies than others, and I'll go to the ones that value me more highly, thank you very much.
*This story is also a perfect example of things recruiters do to manipulate you. She complained about their costs, and made vaguely threating noises about other benefits. There was a general air that I was horrible person for bringing it up, and was quite possibly taking food out of starving children's mouths. I suspect that if this were actually the case, they wouldn't be quite so eager to administer my next contract.
**On a reasonable scale. Few jobs are so odious I wouldn't do them for a million dollars a week, but even fewer jobs are actually worth that to the company.