Selling yourself should never be your focus when you walk into an interview. You've either done the legwork or had someone do it for you to respond to the job opening. You've meticulously prepared your résumé or CV to presentation-quality standards. You have prepared yourself to make the best possible first impression by dressing appropriately for your introduction to your new potential supervisor or manager. The "selling yourself" part of the process is done, but there is an even more important task you need to accomplish once you get the interview.
If this is a job you want with a company you want to work for, you need to find out as much information about that company as you can. You need to determine whether this company is in alignment with your long-term career goals, your personal philosophy, and if their product or service is something that interests you deeply on some level. This is the difference between simply finding a job and advancing your career.
As soon as you sit down in the interview and start discussing the job, your focus should shift to selling your interest in working for this company or organization. Especially when you are applying for an IT position, your ability to communicate and convey your interest in the company over and above the skill set required to simply develop and maintain their systems will be noticed. You need to communicate, by your questions, that you are not just looking for a paycheck. You need to show them that you plan to be there for the long-term, and that you are actively interested in contributing to their success. It's sometimes the difference between simply being an "in-house contractor with benefits" and being given the opportunity to advance in the company.
Hiring managers, especially in the IT field, don't like turnover in permanent positions. If they wanted that, they would hire contractors or outsource. They want to make sure that the people they hire want to work for them, believe in what they do, and can provide a positive return on their investment. Any coder can simply write code. What the managers tend to look for more often than not is someone who can actively demonstrate their willingness to take ownership in the projects they work on, because they themselves are invested in the company's success, as well as their own.