New clients, referrals, sales and training clients... yikes! That is a lot of pressure. But you have the skills to make it happen! Making a routine of spreading the word about your personal training while working with your clients will help you to maintain a steady stream of new clients. This questions indicates that sales are important, so you will need to show the interviewer your enthusiasm and ability to handle the pressure of meeting sales goals. Consider giving an example like this:

"I perform well under pressure and have had to meet sales goals in the past. I take time to promote myself daily while providing quality care to my current clients. Consistency and willingness to be persistent helps me to get more referrals and continue to grow my clientele."