What are your secret must-work tips for a successful job interview?

Be on time!
Dress appropriately
have an updated resume ready and available
be well informed. do some research ahead of time so you are able to ask questions and show that you are informed about the position.

Usually those who get interviewed are more or less equal in the qualifications area so the factor that will make the difference is attitude. Since no business will exist long without satisfied customers you should impress upon the employer how "customer friendly" you are. Be sure to convince them that you are flexible (regarding work schedule) and a team player as well.

Self confidence and a dazzling smile! Smile says "confident" so flash it - even if you don't feel it!! (Just don't overdo it so you look foolish. . . !)

It's all about presentation. Dress smartly and conservatively. Try to stay calm and don't be excessively pushy unless the job requires it.

do your homework abt the company, research them so u have some background info, will seem that u already taking an interest in them. self confidence; be yourself, have a sense of humour, ask and/or clarifiy anything u dont understand; dont chew; maintain eye contact; dont over talk/over sell yrself, let your achievements speak for yrslf; when asked questions- if u never had such an experience, answer with what u would do if in such a situation, never say i never had such an experience; dont repeat yrslf; ask questions that matter most to u example money; position; etc. U dont want to be unpleasantly surprised when an offer is made and you are totally in the dark abt the position and salary etc

Neat hair, clean fingernails, clean shoes. Dont wear too much make-up. Have a pleasant attitude without being so happy that people hate you. Dont bring up negative things from the past. Be willing to learn. Carry a nice pen, I dont hire people that did not remember to bring a pen.... just me.... pet peeve.

Pretty much all good advice here I'll add a few I've looked for when I was a hiring manager.

1.) Keep your answers short and on topic. This isn't a 'shoot the breeze' situation.

2.) If you don't know the answer to a 'skills' type of question say so... nothing worse than 'muddling through' a question you clearly don't know the answer to. Simply say "that's an area where I've always wanted to improve my knowledge/ability/experience... would this position give me an opportunity to do that?"

3.) Attitude you need to 'put forth' will vary depending upon the type of job.

Sales - Aggressive go getter who understands that keeping an existing customer's satisfaction high is at least as important as bringing new customer's on board.

Technology Services - Calm cool professional who's been there and done that.

Management - Organized and efficient with a strong focus on team building skills and understanding of the company and it's industry.
Etc.

4.) Be prepared to ask at least a handful of questions... at least one of these should very specifically show you researched the company prior to the interview.

5.) Make sure you ask about compensation before they ask you what salary level you are looking for... if you ask them first the number they give you is a minimum number you can negotiate up from. If they ask first the number you give them is a maximum number and you can only negotiate down from...

Good luck!