I've found this following method very good:
You can leave the battery in, that extra weight does not add to the shipping price.
If there is no hard disk, just put the HD-cover in (if available).
Wrap the TP tightly in bubble wrap (big bubbles preferred) with enough extra on the short sides,
so that it will just squeeze in the standard flat Priority Mail Box O-1097
Dimensions: 11-1/2" x 13-1/8" x 2-3/8"
Put in an extra rolled strip of bubble wrap to fill any leftover gap along the long side. Close the box.
Put this box inside the slightly larger flat Priority Mail Box Flat Rate Box-O-FRB-2
Dimensions: 11-7/8" x 13-5/8" x 3-3/8"
It's a tight squeeze, but it fits.
This laptop now has an extremely sturdy packing, and there is really no need to use special laptop boxes (unless you have a money tree in your garden).
Both boxes are available for free from nearly all US Post Offices.
[Note: if you are an eBay shipper, you can order those boxes online free in packs of 10 or more, shipped free to your home].
You can ship the laptop now via Priority Mail Medium Flat Rate to:
Anywhere in the USA: $10.95
Canada and Mexico: $27.95
Rest of the world: $45.50
[EDIT: prices correct as per 11/4/2011
Don't forget to insure the package!
Ship the AC Adapter and whatever else you have in a separate small box.
For details on available boxes, see this USPS webpage