Hi Wendi,

Is she able to tell her teachers why she's sad?  My son went through a little phase of this in preschool and he would tell his teachers he missed me (heart breaking!).  They asked us to bring in a photo of our family which they put up in his cubby.  They also had us bring in a special "lovey" (stuffed animal).  This part is going to sound a little strange but they asked me to give the lovey 100 hugs so that it was "full of mommy's hugs" before we brought it in.  Then, any time my son got sad and missed me he was allowed to go to his cubby and see the picture and get one of the hugs from the lovey.  It really worked!  He used it about 6 times and then just stopped getting sad.  Maybe (if your daughter's getting sad because she misses you) something like this could help.

Another possibility might be that she gets hungry at different times that the school has snacks or meals.  (My kids often have an emotional melt down when they get hungry).  Maybe you could keep a few pre-packaged snacks in her cubby in case she needs an energy boost to keep her going.

If she just seems to be having these little "melt downs" out of habit maybe a star chart could help.  (Keep a chart at home and tell her that every time she makes it through the day without crying at school she'll get a star.  Then when she gets 3 stars she gets a prize like a new book or going out to lunch with mommy or something).

I hope these ideas help.  Let me know how she's doing!