Make sure you create a "My
Disneyland" account on the DLR website first, and that you are signed into it.
Then go into the Planning/Hotel section and plug in the dates at the hotel you want. Wait for the system to generate your "offer," which is really just the rack rates or whatever general discount they may be offering to everyone.
Somewhere at the bottom of the offer there will be a tab to "Save" it. Click Save and it will be stored in your My Disneyland account. You can even go into your My Disneyland account and double check your Saved Offers to be sure it's there.
Then leave the offer there and don't do anything. Don't book anything. Disney will see that you showed an interest in staying onsite and most likely send you an e-mail or regular mail with a PIN code.
EXCEPT for the fact that now, more and more reports are coming through that people are getting e-mails without PIN codes at all! Or, like aurorae, you might get a PIN code for WDW instead of DLR!

Or you might get a PIN that does not cover the dates you want (like mine!).
At first, this trick was working like clockwork, almost. But I suspect that there were only a certain number of PIN codes to go around, and DLR has probably reached its PIN limit on the rooms attached to this offer.
So you may have to wait for this current PIN wave to cycle out and expire and then try saving something. I have a feeling that a lot of folks who save their offers now is going to either get a PIN they cannot use or no PIN at all. We may have to wait a month or two for some of the current PIN's to expire.