I personaly always have 2 lessons with each trainer I am thinking about because in the first lesson the trainer is only getting to learn what level you are at and how far/fast they can push you. In the 2nd lesson you might actualy get something done and learn something.
I've found that I like pushy, stroppy trainers who call a spade a spade and will get cross with me.
I work best in that environment, my current trainer has the motto that if you can still stand upright after a lesson then you havent been working hard enough! Several times I have gotted off a horse at the barn and buckled at the knees, then I've gotten into my car and wondered how the hell I was going to stay awake long enough to drive home! However I always always walk away from these lessons feeling that I've achieved something, often havig achieved something massive. I like working hard and seeing results.
That said some people cannot stand my instructor and they don't learn anything from her because her style of teaching is deffinatly not for everyone.
I've run through hundred of trainers before settling on the one that got the best results for me!!