When my son went to preschool, putting a "kiss in his pocket" worked (he kept his hand in his pocket as he walked away from me), but when my daughter went 3 years later, it didn't work (she's much more attached to me and would cry whenever I left her anywhere). We went shopping together and bought a special (inexpensive!) pendant that I would 'fill with kisses' every morning. When she would get sad (her reason for crying), she would take it out and rub it on her face to remind her that I loved her and would come back to get her. You might do something similar so she has a "piece of you" at school.