In my experience as a Lego seller, I have found the best way to ship bulk Legos is in a large plastic bin such as Rubbermaid.  You can purchase these inexpensively at places like Walmart or even dollar-discount stores.  Just make sure it is the type of plastic that has some bend to it as opposed to hard plastic that will easily crack. 

Just stick the label on the lid and wrap bin and lid together using shipping tape.  Place tape horizontally and vertically around the bin, wrapping it several times each way.

The second best way, though it has been known to fail, is to pour the Legos into a large, black yard trash bag.  Tie it securely in a knot at the top, then place it into a large, sturdy box.  Wrap the box horizontally and vertically several times with shipping tape.  The reason for placing the Legos in a bag is another measure of protection if something happens to the box while in transit.

Keep in mind, that when using USPS, Parcel Post is not always the best option, though it is the cheapest.  Any packages you ship using this service will be placed in large bins with other packages up to 70 lbs.  That's a whole lot of weight to withstand.  Since I switched to Priority, I have not had one damaged package.  It is definitely worth the extra money.  Of course, requiring insurance is a must when shipping bulk Lego.