Part of this goes back to the last issue, not having a well-thought out interview process. Once you have the process set out, it is easier to start thinking about what questions you would like to ask at what stage in the process and then to integrate those questions into your process.
I am a huge believer in asking open-ended interview questions, and there is much more information on this in the article “Open Ended interviewing”.
There are a number of questions that it is illegal to ask during the interview, but as a rough rule of thumb anything that would limit the candidates ability to carry out the functions of the job is something that is perfectly appropriate to question.
I have a client who, I am ashamed to say, seems to use the 10 mistakes as his own personal checklist to make sure that he made each one in turn. After panicking that his secretary had resigned and given him no notice, he advertised on craigslist, started interviewing people the same afternoon and hired the first decent candidate he found. Three months later, when she was finally becoming productive, she told him that she was leaving to go to school -- something he had never asked about in his hurried and poorly thought out interview "process".
It is very important to identify what the candidates agenda is to see whether they are really a good fit with the job. This is more true in a down economy than at any other time because when people are desperate they will take a job that is way beneath them. When the economy turns around they will start looking for something that is commensurate with their true abilities, and that is the time when you are likely to lose them.