I've been pretty lucky, I had my first 2 interviews when I was young and lied my head off. I didn't get the first job because I was early so I killed time by buying a double egg fried sandwich from the local cafe, I bit into it and the egg shot all over my tie and shirt. That put me off and I looked a mess. I got the second job along with a company car, I didn't have a driving license! Later in life I interviewed plenty and I'd be very surprised if many were 100% honest.
As others have said, you must make sure everything written to them, is well written and has zero spelling errors, I mean, if you can't be bothered to check..............
Don't put any obstacles in the way of the employer, like travel issues, holidays booked, days/times that you'd prefer not to work, often leading questions are dropped in to illicit such responses.
Don't leave without saying clearly that you really want the job.
If there are local companies that you want to work for, then walk in and tell/ask them, I've seen jobs won this way
If you don't get the job and really did want it, then follow up with a letter saying how pleased you were to meet them, thanking them for the reply, and telling them you would appreciate it if your details were kept for if the vacancy arose again. You may have been 2nd/3rd choice. I've given jobs to people who did that rather than go through the interview process again which is PITA for a small business.