Often, the missing ingredient with someone not achieving their fitness goals is improper nutritional habits. Typically, trainers are not registered dieticians, but if they have kinesiology degrees, they likely have had nutrition classes. Again, this applies only if you're interested in training for a particular sport. Proper sport-specific training often is beyond the scope of many personal trainers. Proper insurance coverage would protect you and your trainer in any lawsuits.
Many trainers offer a discounted rate for payment received in advance.
Pre-payment also tends to improve exercise adherence. Sometimes trainers require a minimum number of sessions before they will accept you as a valued client. You must be sure that your personal trainer will be available at times convenient for your daily schedule. Other issues like personality traits and integrity will surface the more you work with your trainer. Remember, it's a two-way street.