WARNING: this technique may be dangerous.
For a great effect on dioramas or damaged models, try lighting something on fire. A favorite material of mine for dioramas is gauze pad, which I use for camo nets and other things. Recently, I finished a diorama featuring an Su-122 in the middle of a bombed-out street in Russia. I put gauze camo nets over some parts of the vehicle, then lit them on fire to make the vehicle look like it had been hit by a German flamethrower unit. I also used fire in the diorama to warp window frames in the damaged house section and to twist wreckage from an M3 scout car. If you have any cardstock objects in a diorama, light them on fire for a second to make them look damaged.