This is no time for true confessions of major or even minor problems.

BEST ANSWER:  Again never be negative .

Example:  "I suppose with the benefit of hindsight  you can always find things to do better,  of course, but off the top of my head, I  can't think of anything of major consequence. " 

(If more explanation seems necessary)
Describer a situation that didn't suffer because of you but from external conditions beyond your control.

For example, describe the disappointment you felt with a test campaign, new product launch, merger, etc., which looked promising at first, but led to underwhelming results.  "I wish we could have known at the start what we later found out (about the economy turning, the marketplace changing, etc.), but since we couldn't, we just had to go for it.  And we did learn from it."

Never badmouth your previous industry, company, board, boss, staff, employees or customers.  This rule is inviolable:  never be negative.  Any mud you hurl will only soil your suit.

Especially avoid words like "personality clash", "didn't get along", or others which cast a shadow on your competence, integrity, or temperament.

BEST ANSWER:

(If you have a job presently)
If you're not yet 100% committed to leaving your present post, don't be afraid to say so.  Since you have a job, you are in a stronger position than someone who does not.  But don't be coy either.  State honestly what you'd be hoping to find in a new spot.  Of course, as stated often before, you answer will all the stronger if you have already uncovered what this position is all about and you match your desires to it.

(If you do not presently have a job.)
Never lie about having been fired.  It's unethical - and too easily checked.  But do try to deflect the reason from you personally.  If your firing was the result of a takeover, merger, division wide layoff, etc., so much the better.

But you should also do something totally unnatural that will demonstrate consummate professionalism.  Even if it hurts , describe your own firing - candidly, succinctly and without a trace of bitterness - from the company's point-of-view, indicating that you could understand why it happened and you might have made the same decision yourself.

Your stature will rise immensely and, most important of all, you will show you are healed from the wounds inflicted by the firing.  You will enhance your image as first-class management material and stand head and shoulders above the legions of firing victims who, at the slightest provocation, zip open their shirts to expose their battle scars and decry the unfairness of it all.

For all prior positions:
Make sure you've prepared a brief reason for leaving.  Best reasons:  more money, opportunity, responsibility or growth.

TRAPS:  Your interviewer fears you may leave this position quickly, as you have others.  He's concerned you may be unstable, or a "problem person" who can't get along with others.

BEST ANSWER:  First, before you even get to the interview stage, you should try to minimize your image as job hopper.  If there are several entries on your resume of less than one year, consider eliminating the less important ones.  Perhaps you can specify the time you spent at previous positions in rounded years not in months and years. 

Example:   Instead of showing three positions this way:

6/1982 - 3/1983, Position A;
4/1983 - 12/1983, Position B;
1/1984 - 8/1987, Position C;

.it would be better to show simply:

1982 - 1983, Position A;
1984 - 1987 Position C.

In other words, you would drop Position B altogether.  Notice what a difference this makes in reducing your image as a job hopper.

Once in front of the interviewer and this question comes up, you must try to reassure him.  Describe each position as part of an overall pattern of growth and career destination.

Be careful not to blame other people for your frequent changes.  But you can and should attribute certain changes to conditions beyond your control. 

Example:  Thanks to an upcoming merger, you wanted to avoid an ensuing bloodbath, so you made a good, upward career move before your department came under the axe of the new owners.

If possible, also show that your job changes were more frequent in your younger days, while you were establishing yourself, rounding out your skills and looking for the right career path.  At this stage in your career, you're certainly much more interested in the best long-term opportunity.

You might also cite the job(s) where you stayed the longest and describe that this type of situation is what you're looking for now.