Never bad-mouth – you may of be very miserable in your current job but it is really a no go zone to talk about your boss in a negative light. Be positive and turn the negative experience into one where you have learned from and show the employer this in your answer.
It’s not about you – describe the lack of growth in the organization instead of “I need more money”.
Practice your answer – come across as open and relaxed with your answer not one where you a sweating profusely.
Finally, always answer this question with grace and maturity will be the best way to impress the interviewer.