Take the time to identify your client base and then take more time to understand what motivates them, this is where specialisation will really help you focus and come to understand your clients better than if you just choose to be a general personal trainer and will lead to more financial success.
Look for patterns in your chosen group of people. Go on public forums where your target audience dicuss their success and problems. Areas where you will want to focus on when seeking to understand your client will be:
What is success for them? What is their goal.
What is their strength?
What are their weeknessss - potentional problems?
What are the usual sticking points?
What are the usual platuas and how did they overcome them
Common injuries
Next write down key phrases and words that relate most to the profile of your target client. Write down a whole script and routine for the most common situations, questions, opinions, and problems. When you come a cross a situation that you don't have a routine for take the time to create one. Know your scripts off by heart.
Taking the time now to build a profile of how to influence your personal training client will reap great rewards later and save you time. You will be propared for most eventualities and should have a response for most situations.
Have a script and routine and practinig it as often as possible will give you confidence when you speak it. And confidence is a huge part of the job to motivate your personal training client. It will make it easier for your client to belive in you and have confidence in what you say and why they chose you.