If you know that she is really doing fine at school and it is only due to the initial separation, then I would recommend a book called "The Kissing Hand." I read this to my daughter when she was in early elementary (recommended by one of her former/favorite teachers). I would also kiss her hand and leave a lipstick mark before she went to school. The lipstick would wear off so I went a step further and gave her a little sticky note with my "kiss" on it (put on lipstick and kissed the paper). I told her whenever she was missing me to look at the paper and know that I was giving her a kiss.
For us, it turned out that the year she was having so much trouble separating from me was due to problems with the teacher she had and it sent her into an actual depression. She's in college now, but still talks about what an awful year that was and how it affected her for years to come (and I'm not talking about abuse either).