In general we do it by overlaying stock footage (there's tons of it on the usual agency websites, for a price). If your 'damage' clip doesn't already have an alpha channel, change the blend mode in the clip's Opacity effect (multiply, overlay, etc), then play with the opacity slider until you get what you want.
A few people I know shooting on DSLR will use this idea to put back the "film look" - running filmscan on a strip of blank developed 35mm cine then looping it over their digital footage to put back the extremely-slight artifacts that you get from shooting on film. There are plugins for the same thing, but the DIY approach is cheaper if you can get hold of the stock.
Effects that I do not have to buy?
If you ever decide to, Magic Bullet makes a great one called Misfire. It comes with Looks, which itself is something I consider a 'must have' plug-in.