An excellent Jim Henley post in which he explains that he's a "my sister" libertarian:
Nobody worth performing the Heimlich Maneuver on is going to tell the police they saw their sister smoking pot. Am I okay with my sister going to jail if she sells some pills or her favors? Do I think my sister or brother should be dragged into court if she drains her field or he hires too many people of the wrong color? No. So I have no business supporting a regime that subjects other people's siblings to those things. Would I have to agree that if my sister drowned my niece, or my brother defrauded credit card companies or my mother burned down her building for the insurance, that they should be subject to arrest and imprisonment. Yes, I'm afraid. And a note to you smartypants readers: Not all of the examples in this item have been hypotheticals. So I really do mean it.
Whole thing's worth reading. Me, I was ruined by analytic philosophy, so I have a weakness for elaborate justificatory schema involving four stage processes and bargaining positions embodying conceptions of justice. But I've got my own rule of thumb as well. Orwell once wrote of W.H. Auden (a bit unfairly) that his "brand of amoralism is only possible, if you are the kind of person who is always somewhere else when the trigger is pulled." Well, call mine the "you hold the gun test": Would you make X a crime if you had to go and get the violators to cough up the fine or accompany you to jail at gunpoint? (Assume this is safe for you, since otherwise I suppose you might get a perverse result re: the most violent…)
ADDENDUM: This is actually an old post I noticed because Catallarchy recently linked it.