What to do when you have "seller's regret"

Saw a interesting question on a job discussion board today as follows:

"Just found out that this guy who was hired few months before me makes 4k more than me. We are at a same level and came in with about the same experience. However, he did make more than me at his prior job. Now the question is should I be upset at myself for not being able to negotiate my salary to its fullest, or should I just accept that he got the higher offer because he was making more at his prior job?"

One person answered: "just negotiate again when you are up for a review/raise."

To which I added:

And when that time comes, do your homework on

1. What the market will bear for the job in question (try salary.com, talking to people in similar jobs, or other on-line resources for information - see http://www.gccoach.com/articles/SalaryResearch.htm).
2. If your company uses a salary grid, what the salary range is for your job grade.
3. What is it about your performance, potential, or responsibilities for the coming year that might justify an above-average increase.