What concerns me (not from a personal standpoint as I am far away from the danger here) is that if this guy (or gal, although I doubt it - rare profile) has done his homework and prepared well, he should be able to get away with this nearly indefinitely.

Pick random targets, scope the area first, use different modes of transport, avoid looking shifty (technical term) and don't get greedy.

Although what invariably happens in the nutter sniper cases is they tend to get crazier, or more reckless and that's when catching (or at least finding) them becomes easier. Either that, or a friend or relative will suddenly realise what he's been up to and let the world know...probably for a big cash payout from a major newspaper or news network.