This is mainly for me:
Maverick, Lucid & Karmic users can add it via the following command below. Note that it will uninstall any 32bit version of Flash you have installed prior to replacement.
sudo add-apt-repository ppa:sevenmachines/flash
sudo apt-get update && sudo apt-get install flashplugin64-installer
Installing 64bit Flash in Ubuntu using a .deb
If the idea of adding a PPA is a little bit excessive for your needs you can, of course, just download the relevant .deb file from SevenMachine’s PPA instead.
Easy links below. (2013 edit: the below links no longer worked, have removed)