As you might have read, all you need to do is search in the Store for the application: Preview for Developers, install and open it. It opens the door to the set of updates that will trigger the installation of Windows Phone 8.1.
The thing is that one of the steps when opening it require you to enter your Live Account credentials, and in theory you must be registered as a Developer in the Windows Phone Dev Center.
Sincerely, you should consider register as so if you plan to do any development... It's only $14 yearly... But if you don't want to pay, there is a way:
1.- Go to http://appstudio.windowsphone.com
2.- Log in with your non-developer Live Account ID, and accept all the terms and conditions
3.- Click in "Start New Project"
4.- Select whichever project template you prefer (empty app will do the trick), and complete the process.
Now, you should be able to run the Preview for Developers thing, and it should accept your credentials as a registered developer.
Ta dá !!!
Disclaimer: Do this at your own risk. Windows Phone 8.1 is currently a developer preview and might contain bugs. I take no responsibility for any damage done to your phone or to your data, so better know what you are doing... :)