Brent Simmons — creator of NetNewsWire, Glassboard and Vesper app — has an excellent series called "How Not to Crash", featuring excellent tips such as this:

It should be extremely rare — or, better, never — that an object has a public property that is a mutable collection. Mutable collections should be internal to the object.

There are many gems in there. Go check out "How Not to Crash".