Salary, at least in the (several) parts of the US where I've lived, correlates to negotiating skill as much as to job skill. Both are important; a good programmer usually makes more than a poor one, but a mediocre programmer who is a good negotiator will make more than a good programmer who is a mediocre negotiator.

People in engineering and technical professions hate hearing that, but it's true. Learn some simple and basic negotiating skills. Even if you don't want to use manipulative techniques on others, at least learn to recognize when you are being manipulated and what to do about it.

Never go into a salary negotiation without a figure in mind. Web sites and industry magazines are good starting points for information on what to expect. Try to get your (prospective) employer to name a figure first, though, in case you're pleasantly surprised :)

Once they make an offer, ask for it in writing (an offer letter) and don't discuss anything else with them until they send it to you. Get a copy of the employee handbook so you can review benefits and check for gotchas. Keep a few recent offer letters and use them in other negotiations; when I moved to Denver, I got a high offer from a company which would have been my last resort, and waved that letter at the president of the company I did want to work for, which got him to raise his offer by 12%.

Walk away from mediocre offers or situations that make you uncomfortable. Don't be rude about it, but make it clear that you are walking away. Some will call you back with more. When I was interviewing in Oregon a company offered me a dream job with the catch that I was expected to put in 70 hour weeks. I told them clearly that I would love to work with them, but couldn't agree to that and went home. The next day they called and offered me the same job for 40 hours a week.

As with anything else, this will improve with experience. I was reamed ridiculously at my first job, but handled the situation better with each move.