How do I keep the slats in our bed from falling out?

Our bed has wooden slats in it and when the slats were bought they were mesured for the bed, but I am not sure when they were purchsed, I got the bed from my parents and they have had the bed for a while. The slats fit when we put them in and we are not sure why they fall. How can I keep the slats from falling out. We still live in an apartment and will move often until we are able to get a house or put this bed in the guest room so I need something that can be easily taken apart! Thanks for any help!