What is best way to sound proof between units?

[Brian Cremer]

I have an old house that is a 2 unit building, one up and one down.  It is currently down to the studs and I am replacing all mechanicals and windows.  I have seen some threads on how to finish ceilings to eliminate noise transfer between units, but it has been awhile.

I was wondering what techniques you have used to insulate and finish drywall to cut down on noise between units.  All suggestions and links to blogs, threads and products would be greatly appreciated.

Thanks!