Continued.
Do not ever say "good question" untill you are an expert highly sought after. It is not as bad as "what a stupid question!" but not by much.
You can say "nice question, I'd like to answer that" meaning "thanks for asking that - I really know this stuff, now sit back and watch me shine".
I can't stress it strong enough - people
will ask you stupid off-the shelf questions and will even expect off-the shelf answers, but that does not mean those people are stupid or will disregard what you've said or how you've said it when teh decision is made.
You will probably be asked something like "what was your
life's biggest challenge of which you are most proud".
You can say "programming X, Y and Z, even if it took you weekend". But more realistic answer would be "I understand you are referring to professional career, and that would probably be implementation of X,Y and Z, and career is very important in my life, no mistake about it - but the most challenging event in my whole life was probably staying alive through a bungled counter-ambush operation after our planes accidentally dropped napalm on our position and ammo resuply on enemy's..." Or "rasing good children".
If asked, why do you want to work in this company, do not give them crap "I've studied all I could about your company and can only work here". They know it's crap, the job market is tight.
Anayway, you work not in the company but in a group or a department and those can be hell in an otherwise great company. So tell them you are a professional for hire. Make them a compliment that their's seems a nice environment (group, office) where you'd love to work. They seem to have growth opportunities for you - indication you would not jump the ship righ away. This kind of stuff.
You
may thoroughly prepare prepare in that respect as Dowding suggests - if you have nothing better to do. But for God's sake, do not spit that noncense out indiscriminately. Mentioned that
you've learned a whole lot about their company but do not go into details - it will turn most people off. Most people will assume it's a meaningless politeness and forget about it. Very few who care will surely ask you to elaborate - "what did you learn". Only then start the ass-licking. Most likely that would be an HR person or some managing director on business side, not technologist. Most techies will be repulsed by it. The technologist wants grunts to do work, not politicking smooth-talkers. Most of them did not do this kind of stuff themselves - even if they think it was worth doing they may hold it against you.
If you talk with someone from personell/human resources - not professional, talk more about credentials. Do not confuse them with technical details.
If you talk with a professional, emphasize experience/abilities, not credentials. He may have got worse grades than you or less prestigious college - no need to make him resentfull.
In american IT I would advise you not to send "thank you" letters to anyone, or if you do - only to the boss responcible for hiring. Do not make it too elaborate and personal, like "what a great team he has". You are not desperate. You just normally do that kind of polite stuff without much thought. Mail it three days later, not the next day.
Do not call them in a couple of weeks and do not promice that - you will be expecting a call, not making it.
There are all kinds of wacky people out there and you would never fit all their peculiar preferences. For every guy who would not hire you without a follow-up letter, ten would not hire a stiff shirt who would send one.
BTW, you can ask an underling, if the boss will expect one or be scared by it. They know you are hman and want a job. If they like you, they will give you a good advice. If not - you have nothing to lose.
Most managers are former technical people not very good at administration or interested in it. Some things are not a sign of neglect, just of chaotic organisation.
You do not know their situation. Just forget about it and concentrate on what you can affect. You can be fifth guy on their list but the first four may decline offer or quit after two weeks.
Joke some (1-2 times), not a lot even if they seem to like it.
If offered a low salary, say that you would prefer more but your financial situation is such that you can take a lower paying job in such a great company

because you are sure after the first year they will be glad to pay you more - once they realise how good you are. That may result in immediate raise or not but will not drop you from consideration.
They may have relaxed athmosphere and you can express your appreciation but a suit and a tie is what you wear for an interview. It does not have to be a great tie - you are a techie, after all - but a standard whole and clean one. 3$ get you one from a chinese peddler in the street.
Try not to say crap like "you see...", "like...", "uhhm". Practice beforehand but if you can't control it, do not worry. You are not interviewing for a network ancor. You cannot do too many things at once. Better be relaxed and as natural as possible.
Oh, treat an interview as a free education for the future one. Note things. That will take your mind off from worry.
Have a notepad or a hidden recorder. As capt.a. said, note names, departments, relationships - who is who, does what, works for whom.
Good Luck.
miko