When I was a kid, when I'd wargame, I'd knock the (plastic) figures down to indicate it had become a casualty. Of course, knocking down 15mm painted metal figures (on bases no less) is not an option, and I always felt something was missing from my skirmish battles.
The thing that was missing was body markers.
I mount all my figures on individual metal bases, so I can group them on larger magnet movement trays as needed for whatever rule-set I want to use. This also allows me to remove figures as casualties. This system is flexible enough to fit all my needs.
I used 3/4" magnet strips (magnetized on only 1 side) cut to the size of 1 figure. On the magnet side is a color (for which side the figure belongs to) and unit ID (to seperate the unit). The side color is important because a lot of figures (especially 15mm) look alot alike when all massed up and fighting one another. On the backside is a depiction of a horse or man (dead of course).
With the small movement bases the figures stand up (the metal base is pretty small- I wanted as small a footprint as possible to allow maximum flexibility on my movement trays) easily, and the figure clings to the base. When the figure dies, I remove the figure and flip the base. Voila ! Instant casualty marker.
After the battle, you can see the course of the action by the trail of casualty markers. It kind of adds a conversational centerpiece when cleaning up for the next battle !