If you understand what is really being asked when you get a tough question then you can answer to your best advantage. Knowing what a questioner is actually investigating is the secret to a successful interview.
This is a question which trips a lot of people up. They attempt either stark honesty and reel off along list of negative personal issues or they deny having any weaknesses at all. A better approach is to pick a weakness and tell the interviewer how you have overcome the handicap.
"I have a natural tendency to stammer so 2 years ago I booked myself in for speech and presentation lessons and I am happy to say that my colleagues in my previous role noticed a marked change in me as a result. i am now far more confident about public speaking than I was in the past."
This question is actually asking "are you serious?" The response being sought is that you have thought about this application and have already decided that you want to work for their company.
"Your company has a public reputation for excellence in the fields of x, y and z and I want to be associated with that. I have applied for organizations which I believe will offer me an enjoyable, stretching and ultimately rewarding employment. When I saw your advert, I then went to your web site and phoned through to receive your marketing literature and find out more about the company. Everything I have heard so far makes me very excited at the prospect of joining your team."
Be positive. Always state your reason for leaving in a way that puts you in a good light. Do NOT bad-mouth your previous employer.
Employers are looking for ambition tempered with realism, so a bit of vagueness is OK if you combine it with a positive.
"My immediate goal is to work in an organization such as yourselves where I can develop my x, y and z skills and experience. My long-term goal will depend on the directions the role and organization take. I hope to eventually land a position of responsibility."
A difficult one. Yo don't want to price yourself out of a job or to undersell yourself. If possible turn the question back on the interviewer, getting them to tell you the range first. If you cannot do this, then have a decently high baseline figure in your head and indicate adaptability.
"I am sure can agree on an amount we are both happy with. In what range do you typically pay someone with my background?"
If you are looking for more support to get through tough interviews there are some great books about interviewing techniques available.
For example, Prepare for Test at Interview by Robert Williams is really good for anyone looking for a graduate or managerial position who has been told they will sit an ability or personality assessment. It provides great advice and examples so you know what is coming.