I admire you for seeing this as a problem and your willingness to do something about it. I know many parents who see nothing wrong with praise being lavished on their children. In fact, they eat it up as it strokes their own egos.
They're unaware of the research that shows too many compliments make kids overly dependent on adult approval. Children lose the ability to evaluate their work and need moms and dads to tell them whether it's good or bad. They lose the intrinsic motivation that's so necessary to succeed in life, feel fulfilled, and be happy. They're always looking for validation from the outside, not the inside.
Since some preschool teachers are unaware that too many compliments create “praise junkies,” I would privately and respectfully bring this to their attention. You could mention that you read an article about it in a parenting magazine and grew concerned. You could elicit their opinion on the matter, so they don't get defensive.
When my son attended a co-op preschool, we had a parent meeting on the issue of praise junkies. Since moms and dads helped in the classroom, we decided to develop guidelines for giving constructive comments rather than mere kudos. It got us thinking about how to encourage children in a meaningful way instead of giving fast, easy, and mindless compliments: You look so cute today...your painting is so amazing...your block city is so cool.
We learned (with time and practice) to ask questions that got the kids thinking: Why did you decide to wear that dress today? What did you feel when you made that painting? How did you build that block city so tall? It was amazing to see how the adult-child conversations at preschool evolved and became more exciting and fun.