The magen david (6-pointed star) was believed to be the symbol that David placed on his shield. I think it was on the shields of his soldiers also, but I am not positive. It would make sense in a way that one could determine whose army the soldier belonged to. Interestingly, in paleo-hebrew (the ancient Hebrew text) the letter "D" looked like a triangle. One triangle with another upside down superimposed on it seems to spell out "D-D" (dwd) and David's name is spelled dalet, vav, dalet.

Hope that helps.