Bingowings said:

Waiting until an overstocked Amazon reduces it's prices however reduces their resell profits and maybe encourages them to stock less Lucas sets in the future, reducing the amount that Lucas takes.

True. Not as much as "just not buying it" though.

Thrift or charity shops are the best option for me because that route has no direct connection to Lucas and you are giving you money to a good cause.

Master Sifo-Dyas said:

Alas, buying 2nd hand (from your neighbour, who doesn't want them anymore, for example), is a different thing.

Lucasfilm still keeps the money he gave them, but they don't get your's - your neighbour get's it. So they sold one copy less.

This is also true. Assuming your friend doesn't replace his/her copy. But you are still taking a unique copy out of the martketplace. Maybe your friend wouldn't actually sell it to anyone else, so it's either they keep it or you do. Doesn't seem to really matter in this case. Unless you encouraged the purchase in the first place.

The charity shop doesn't solicit specific goods- so that doesn't actually create a market pull to get more copies- but again it still takes a copy out of the market, increasing demand.

I meant to say this earlier, but there seems to be a bit of a double standardTM here:

1. Buy the O-OT used and then you've "given LFL your money" to be able to download preservations or fan-edits.

2. Buy the BDs used and you haven't given LFL your money. Screw 'em!