Sprites on a tile based map.

I''m working with an iso-tile based engine. Everything seems to be going to plan, but in thinking about future problems, I have this main one: I want my sprites, which are soldiers and such, to be able to have free movement (not tile based) on the land. How should I keep track of which soldiers are infront of who? I understand about drawing my terrain from front to back, but the soldier are a little trickier since they can swap who is further infront and who is further behind each other. Please help! -Moquan