Hello,
I am a preschool teacher and I have some thoughts for you that might help.
First, when giving praise to preschoolers, "good job" just is not enough. They need and like to hear something more specific like "I like the way you put all the toys in the bin without me even asking!" or "You were a good friend to Nyeela today when you gave her a hug when she got hurt."
As far as stickers go, I never make kids "work" for stickers. I pass them out (or ask a child to pass them out) whenever the majority of the class is doing the appropriate thing expected of them. Basically, I give them stickers just for being themselves. I make sure that everyone gets one as special as the next, or give out all the same ones. The kids love the surprise of never knowing when they will get this special "warm fuzzy" in the form of a sticker.
I do use charts and rubber stamps daily to reward positive behavior. When their stamp sheet is filled up, they get to pick a prize out of the prize bin and take their filled sheet home. They get a new sheet right away and start doing their best to "get caught doing good." I come from a background in Special Education, but I tweaked this system to work with my 3 & 4 year-old typical kids as well as a couple who need some extra support.
Any questions about this subject or ideas to make it even better? I am very open to constructive criticism and like sharing ideas.
Regards, "Miss Alicia"