Look for an educated trainer that has a deep knowledge base that extends beyond general strength training to include endurance training, power training and mind/body (i.e. yoga, pilates, martial arts) disciplines.  This type of trainer will have the tools necessary to give you a complete training program that challenges your body in a multitude of ways which helps to promote change and avoid plateaus. Look for a trainer that is a teacher that will guide you through the exercise and educate you on proper form and technique.  A trainer should also be committed to living a healthy lifestyle.

Variety is key.  Fast and sweaty helps you obtain a caloric deficit but it can be very strenuous on the body, doing it all the time could cause injury.  Alternating with a slow and steady day gives variety to your routine to keep your body responsive and also avoid injury.