I never give cash, but I give intermittent thank you gift cards or candy or other things. For instance, on Valentine's, I usually will do a $10 gift card per teacher (2 teachers in the toddler room, 2 teachers in the kindergarten room) and some Valentine's candy. For Halloween, maybe some halloween cupcakes and a $10 gift card. And when we come back from WDW, we usually bring Disney candy or some other souvenir.
I usually do Valentine's, Easter's, 4th of July, Halloween, then at Christmas I get a bigger gift card, about $30 per teacher.
Also, when the kids move rooms (i.e. infant room to toddler room), I give a thank you card with a gift card.
I've done this since our first DS was in day care at 8 weeks old. I'm not saying other parents should do the same, I just really want these teachers to know that I appreciate what they do for my kiddos. We pay a fortune on weekly tuition, but I know the teachers don't get most of that.