Too many people second-guess themselves after an interview. By closing strongly and asking the right questions, you can eliminate the post-interview doubts that tend to plague most candidates. If you feel that the interview went well and you would like to take the next step, express your interest to the hiring authority and turn the tables a bit. Try something like the following:
After hearing more about your company, the position and the responsibilities at hand, I am certain that I possess the qualities that you are looking for in the (title) position. Based on our conversation and my qualifications, are there any issues or concerns that you have that would lead you to believe otherwise?"
You have a right to be assertive. This is a great closing question because it opens the door for the hiring authority to be honest with you about his or her feelings. If concerns do exist, this is a great opportunity to overcome them. You have one final chance to dispel the concerns, sell your strengths and end the interview on positive note.