My preferred method - bubble wrap on the inside of hard plastic cases, thin foam wrap around the outside, placed in to one of the smaller tough bags (flexible but thick card envelopes, no bubble wrap lining). The bubble wrap on the inside helps prevent damage from outside pressure pushing in, so can help prevent cracks in the case, or the little teeth spring thingies from snapping, and the thin foam around the outside of the case is a good shock absorber that keeps the packages under 2cm so they can be posted as a large letter for $2.

If there are any other types of cases, like bi / tri-fold digipaks etc, try placing a bit of a cotton ball on the inside to help support the plastic springs rather than bubble wrap, which might actually damage them by pushing them inwards since closing the gatefold will create pressure (most of the plastic cases can at least be replaced easily, but there's no replacing or repairing damage to cases like these). Foam wrap on the outside, still. 

If there are some more valuable and / or collectible titles, I'd send them fully bubble-wrapped and as a boxed parcel.