If you are going on a job interview, the rule to follow is: Don't sell yourself. Buy them.
If you focus on telling them how wonderful you are and what great things you can do for them, this implies a criticism of their work... as though they are seriously in need of your services.
If they ask about prior performance, resist the temptation to tell them how great you were. Try to find a way to show them what you have contributed. Bring in a notebook filled with evidence. Always let your work speak for you.
You should focus on how much you admire their organization and how much you would be honored to work with them. If they have been having problems, try a no-fault approach... shift the blame to the business climate, high taxes and regulations. If they ask how you might improve things, have a couple of proposals at the ready.
Buying them begins with extensive research into their business, the market conditions they are facing, their policies, and their programs. Do not, however, act like you know it all already.
When the time comes for you to ask questions, use the occasion to demonstrate your respect for them, for the challenges they face, and the creative ways they have dealt with them.
Certainly, do not imagine that the job is beneath you and that you are doing them a large favor by working for them. That is the road to chronic unemployment.
Buying them also means respecting their culture, showing good manners, and practicing decorum. You know already that you do this by proper dress, good grooming, and punctuality. If they keep you waiting, say nothing about it. If you react as though you have been disrespected, the interview will be over before it has begun.
Getting a job has little to do with how badly you want the job. It is not about wishing, hoping, and praying for it. If you say that you really want the job, you will look desperate.
The best way to show that you really want the job is to come in prepared for it. That means you have put in the time and the effort to demonstrate your control of the information that will be in play in the interview. Hard work will take you much further than wishing.
If you have not fully prepared for the interview you will sound rehearsed, as though you are running through your talking points. No one likes being talked at.