My advice - don't wait until you are planning a US trip. If you are a regular traveller go straight to the ESTA registration page and complete the process. You do not have to have a specific travel date in mind and (if I read this right) only have to do it once.
I hope this public service announcement has been useful. Of course I am only writing this post so I have another excuse to use the Recombobulation photograph and tag the word again :)
Update - I made a mistake above in saving that the green form also has to be completed in this new ESTA world. The form (officially called an I-94W form) does not have to be completed for ESTA approved travellers. See ESTA FAQ pdf for more (page 12)
Update 2 - there is some real confusion here. Despite what I found above in the ESTA FAQ I was still required to fill in the green I-94W form on arrival in the US.
(thank you lark is already taken from Flickr for the fantastic photo)