Another Drag n Drop question

OK, on the left I have 5 nav buttons, which when clicked change the draggable items on the right. The middle is the main drop area. I need the user to be able to create a picture by dragging the items from the right, and putting them in the middle. My problem is that when you click the left button and go to another frame, all the previous dragged clips are removed. Does anyone know how to make them stay where they were dropped so the user can create a picture from each of the selections. I have tried to put the draggable clips on different layers and mask the ones on the right, but it didn’t work. anyone got any suggestions or has anyone done anything similar. Thanks.