This happened with my son when he was 2 years old. The Teacher and I talked a bit about it and her best advise was for me to make a short and sweet goodbye. I was to kiss him and tell him to have fun, that I would see him as soon as I was done at work. I hated it! I felt like such a bad mother leaving my son at day care while I did what I "wanted". I would sing songs on the way to day care, tell him how much I would miss him and I was only making it worse!
I thought that I would try what the teacher suggested. I played a tape in the morning in the car, instead of singing and talking to him. After we walked to his room, I stood just inside the doorway, gave him a quick hug and kiss and said good-bye, then walked away. It was hard not to go back when I heard his cries. I waited in the office to see how he would handle it, and within 2 minutes he stopped crying. After a couple of weeks, he didn't even start crying.
I guess my own anxiety was rubbing off on him, and he thought that he was supposed to cry and carry on when I left him.