Legal ways to collect your bill for auto repairs?

I have a friend who opened up an auto repair business a few months ago, and has been victimized somewhat by people who say they want their car repaired and then refuse to pay for it when the work is completed.

This is in California, where in about 1973 the State Supreme Court ruled that the old statutory mechanic's lien was unconstitutional as denying due process. Of course I suppose you could still make the customer sign an express lien. Do you guys do that? If so, do the law enforcement officials honor it?

Any other tricks you can give my friend for collecting his bills? It's got to be legal. And I think that disabling the vehicle so the customer can't take it gives rise to civil liability, doesn't it?

Thanks for any suggestions you can provide.