I agree with Applesauce. Each of those things are important.
I would also add:
1. Have a nice resume. A perfect resume. And format the resume to the job for which you are applying. Changing a word here or there where you can fit your experience into that position might be what gets you the interview. Your resume should be no more than one page. References are not necessary on the resume, but be prepared to bring a list to an interview. Also bring plenty of copies. Memorize that resume.
eta: it has recently been suggested to me that after you have your completed resume, read it backward. You may catch errors you previously missed.
2. Always send a cover letter with your resume. Make sure to spell the name of the company and the interviewer correctly. Call the receptionist ahead of time and ask. You don't need to tell them who you are or why you are calling (usually).
3. During the interview: Although this is redundant, don't dis your former employers. Interviewers know that if you bad-mouth a former employer, you will bad-mouth them just as easily. When asked questions regarding "how would you handle [insert situation here]," use examples if you have encountered that situation before. The two most difficult questions I've been asked are "what is your best/worst quality?" and "why should I hire you?" I've been able to say that my worst quality is that I might be too friendly sometimes - this is not always a bad thing, it shows that you can work well with others, and have a positive outlook. (It has always worked for me.) Best quality - depends on the job you are applying for. The answer to that question that has worked best for me is loyalty. It's not a good idea to ask about benefits during an initial interview. If they are interested in you they will tell you.
4. Finally, as has been mentioned above, send a thank you letter. Go directly home, type it up while the interview is fresh in your mind and make sure you get it in the mail that very day. I've been told more than once that the thank-you letter has been the weight that tipped the scale in my favor.
Sorry this was so wordy, it wasn't my intent. I've had several jobs, since the best way to get a raise in this town is to shuffle from firm to firm. There is a core group of legal secretaries/paralegals that rotate every year or so. Sad but true.
This applies to a professional position however, and if you are only looking for a short-term, summer job, certainly tone it down.
Good luck and keep us in the loop!