We had this same problem with my daughter when we moved her into a toddler bed. (For us we did not want to take the chance she would fall from her crib and definitely didn't want to use a crib tent from a few horror stories we had heard.)
Anyways, we introduced a star chart and explained to her that if she stayed in her bed all night she would get a star in the morning. She got to choose the color star every morning. We also chose a present that she really wanted and told her that once she got three stars she would get the present. (Once she got three stars and a present, she then needed to get 5 stars for a present, then ten, and so on. Eventually just getting the star was motivation enough.)