I've been doing a lot of interviewing lately for hiring new people and one thing that has become abundantly clear is how unprepared most people are for job interviews.
Here are a few handy tips that may seem like common sense to some but amazingly are not done by 75% of the people I interview (especially recent college graduates):
#1 KNOW THE COMPANY. Get on the website and research the company. Make sure you know as much as you can about the company. When I interview people, I tend to do a quick search for them. Do they have a MySpace page or Facebook page or something else that lets me find out a bit about them. You'd be surprised how many people are tripped up when they are caught knowing nothing about our company but I know the name of their dog. 
#2 BE CONFIDENT & ENERGETIC. Believe in yourself. Nobody wants to hire someone that they think will need to be nursed for months. We hire people in order to have LESS work to do. People who aren't confident or energetic are more likely to be the people who sit around waiting to be managed.
#3 KNOW WHY YOU SHOULD HAVE THE JOB. The question "Why do you think we should hire you for this position versus other people who have applied" is a pretty common question. Don't answer "Because I'm a nice person." Sell yourself, make the case. If you've researched the job, you should be able to come up with a good rationale for your unique capabilities.
#4 DON'T BULLSHIT. Male interviewees are much worse about this. Especially young (right out of college) males. It takes a serious amount of arrogance for some kid to think they can actually bullshit us on our field of expertise. If I ask "How strong is your math background? Do you know Calc?" and you say "Oh yea, I'm awesome at calculus!" then you're pretty doomed if you don't know the derivative of 4X^2.
#5 FOLLOWUP. I had some awesome candidates that just didn't followup or didn't follow-up quickly enough. You should followup with any correspendence you had with the company within 24 hours. This is especially true after the final interview. You absolutely must followup with an email after the interview.
#6 HUMILITY IS GOOD. We always ask people to rate themselves on a scale from 1 to 10 on a variety of issues depending on the job. The number of recent graduates who rate themselves a 9 out of 10 or 10 out of 10 (and we tell them that a 10 would be "total expert") is remarkable. There's nothing more deflating for a candidate who has just answered a 10 out of 10 in C++ knowledge or math or general computer knowledge than being quickly crushed by a series if expert questions that they have no idea about. A good answer is 6 or 7 -- even if you feel pretty confident. Unless you have software on the shelves or a book or whatever, answering above 7 reeks of arrogance.
As a side note, also be nice to the office staff. Amazingly, a decent % of candidates will be rude or short with my office assistants thinking their opinions are of know consequence. First thing I ask my office assistants was how polite the candidate was to them.
#7 HAVE A PLAN. Where do you see yourself in 5 years is a typical question. Have some idea of where you're going.
#8 "LIKE" and "YOU KNOW" are not your friends. Drives me crazy when candidates pepper their responses with immense amounts of "Like" and "You know". Females seem much worse about this. Practice speaking and eliminate those two phrases from your vocabular as much as you can.
#9 DON'T BE ASHAMED OF OLD JOBS. Sometimes I'll discover that someone has removed from their resume menial jobs they've had. Don't. Those are very important jobs that show character building. I won't even consider hiring someone who has never worked a "crap job". The kid with the 3.7 GPA is not very impressive if they've never had a job. They'll lose everytime to the kid with the 2.9 who worked through school. People without crap jobs are historically much less reliable.
#10 INTEGRITY IS KING. More so than your job knowledge, employers will look at your integrity and ability to deal with conflict. We're at work 8+ hours a day, who wants to deal with someone who's difficult? Be honest. The worst answer to the question "How have you dealt with interpersonal conflict at previous jobs" is to say "I've never had any." As it means either you have no job experience or you aren't telling the whole truth either of which is doom. We all have disagreements with coworkers from time to time, even if they're mild, it's a question of finding out how you deal with those disagreements.
Hope this helps!