Alphabetically by band and then by year of release. I do not have compulsive obsessive disorder except where the order of music is concerned. Even the cassettes in my car had to be in order in a case. I don't know how people sleep at night, when they have damaged or broken CDs/cassettes drifting around the interior of their car.
I agree, I Have to know where my music is and it Has to be in some sort of order. When it comes to laundry etc. anarchy reigns, but my music collection is in Perfect order.