I've been wondering about acoustic triangulation. They use that here in Phoenix to pinpoint random gunfire on hollidays, and I know the Israelies have had good luck with it in finding snipers.
The 'beltway sniper' does cover a pretty big area, but it seems worth a shot. For all I know they are already doing this, just not making it public.
I might encourage citizens to carry disposable cameras in case they need to snap a picture of a fleeing perp, which might produce better leads than the human eye. I'd encourage concealed carry, too, but that's never going to happen in DC.
Stores mounting panoramic 1-frame/sec cameras that capture their entire parking lot seem like a good idea.
Finally, ballistic fingerprints for all firearms in the US. (just kidding)