If they haven't paid you, haven't paid a bunch of other people, and seem unable to do so...
Don't spend much money trying to get blood from a stone. Write it off, count it against your business losses for the year, and move on. It sucks, but if they simply don't have the money, there's not an awful lot you can do about it - and spending money on demand letters, lawyers, etc, doesn't help if there's nothing to collect.
And maybe ask for a deposit up front next time you've got a big job. Treat it as the most normal thing in the world and see if they're willing to - if they kick back really, really hard, there's probably a reason. I've also done "payment checkpoints" with some troublesome clients in the past - I do up to this point, they pay me for that work, and I don't do any more work until the payment for the previous segment is taken care of.