Having interviewed quite a few people for software development positions, I'll tell you the things I notice and things I look for.

Be honest. If someone asks a question you'd rather not answer, and don't have a half truth for, say you'd rather not discuss it. Someone who has an embarassing past I will still consider, someone who lies to me, I won't. One sentence answering why you won't discuss it is good tho. (I have a 3.5 year gap in my work history. All I have to answer when someone inquires about that is "marital problems" and everyone is satisfied - no further explanation needed.)

Answer questions directly. If you don't understand the question, ask for clarification. I've had a few people that sidestepped almost every question I asked. I don't know if they were intentionally evading answering, didn't understand the question and were afraid to ask, or just had a short attention span. But none of the above reasons endeared them to me.

Be prepared. Have extra copies of your resume, references, school and work history. Bring a folder with a pad of paper and pen in it.

Avoid flashy jewelry or makeup. Anything that distracts from your face is bad. When someone is listening to what you say, you don't want their concentration broken by the swing and sparkle of a big flashy earring.

Balance social and professionalism. An interview is a way to show off your work habits. If I am interviewing someone and all they want to do is share anecdotes or chit-chat, I have doubts about their ability to work unattended - they might spend too much time socializing with co-workers. I want someone who knows when and how to work without needing a babysitter hovering over them. However, if the interviewer is the person you will be working for/with, they may want to get an idea of what your personality is like so they can tell if they'd be able to work with you.

Be prepared, part II. Get a list of common interview questions (from the books mentioned above, or I could PM you the ones I use if you are interested) and know your answers for them before you go in. It will take less thinking time. One of the hardest question to give an impromptu answer for is "Give me two words that describe everything about you."

Remember that the interviewer is not your enemy. You are not competing with him/her. If you are asked a question and you see multiple ways to answer it, say that it is too open ended for you to answer. Ask what they are looking for so you can give a better answer.
If you can't answer a question, say so. Don't hem, haw, or try to bluff. When that part of the interview is over, ask for the correct answer. Someone who is unwilling to admit ignorance and resistant to learning is not someone I want working for me. Ignorance can be cured, and a thirst for knowledge is a good thing.

That's all I can think of off the top of my head.
And remember that just because you don't get a job doesn't mean that you "failed" the interview. If there is only 1 position but 10 applicants, only 1 person can get the job, no matter how well the other 9 interview.
Best of luck.